JP2006038840A

JP2006038840A - Method for grain boundary etching of metal surface

Info

Publication number: JP2006038840A
Application number: JP2005183854A
Authority: JP
Inventors: Yasutaka Wada; 泰孝和田; Shizuma Shintani; 静馬新谷; Masaki Uekado; 正樹上門
Original assignee: Chugoku Electric Power Co Inc
Current assignee: Chugoku Electric Power Co Inc
Priority date: 2004-06-24
Filing date: 2005-06-23
Publication date: 2006-02-09

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of evenly and surely forming a passive film while suppressing excessive metal corrosion of a crystal grain surface, and selectively breaking and metal-corroding a passive film in a grain boundary part without stopping or holding sweep at a re-passivation minimum potential. <P>SOLUTION: A potential is applied while a etching target part of a metal surface is brought into contact with an electrolyte, and the potential is swept upwardly from a natural potential 1 to a return point of an optional value exceeding a passivation potential 2 to form a passive film in the etching target part. The potential is thereafter inversely swept to the natural potential 1 through a re-passivation area and an active state area to selectively corrode the crystal grain boundary part. The sweep is performed at a high rate from the natural potential 1 to the passivation potential 2 to activate the etching target part while suppressing the corrosion of the whole crystal grain. When the potential is returned from the passivation potential 2 to the natural potential 1 through the return point R, the sweep is performed at a low rate at least up to the re-passivation minimum potential 8 in the inverse sweep to promote the dissolution of the grain boundary part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

この発明は、金属表面の粒界を選択的に腐食させる腐食方法に係わり、特に、金属材料の経年劣化による脆化をその金属表面の粒界腐食溝の深さや幅等を測定して評価するような場合に用いる腐食方法として極めて有用な技術に関する。 The present invention relates to a corrosion method for selectively corroding a grain boundary on a metal surface. In particular, the embrittlement due to aging of a metal material is evaluated by measuring the depth and width of a grain boundary corrosion groove on the metal surface. The present invention relates to a technique that is extremely useful as a corrosion method used in such a case.

高温流体に晒されるタービン等のような、高温下で使用される金属製の構成部材は、長年の使用により少しずつ組織変化を起こして脆化し、材質劣化を来していくが、その脆化度の検出法として、特開平５−２２３７２６号公報等に示されている電気化学的再活性化法（ＥＰＲ法）を利用した経年脆化検出方法が知られている。 Metal components used at high temperatures, such as turbines exposed to high-temperature fluids, gradually change in structure due to long-term use and become brittle, resulting in material deterioration. As a method for detecting the degree, an aged embrittlement detection method using an electrochemical reactivation method (EPR method) disclosed in JP-A-5-223726 is known.

この経年脆化検出方法は、電解セルの開口部を通じて対象金属表面の被計測部に電解液を接しさせて、当該被計測部に電位を加え、この電位を増大方向に５mV／secで掃引しながら被計測部と対極との間に流れる電流密度を計測し、この電流密度が不動態域で極小値になったところで当該電位の掃引を一旦停止させることによって、被計測部に不動態皮膜を形成せしめ、この不動態皮膜形成後に電位を同じく５mV／secで逆掃引させて電解液の電流密度の極小値を確認するとともに、この電流密度の極小値を与える電位を一定時間保持することによって、被計測部の粒界部を選択的に腐食せしめ、爾後、その粒界腐食溝の深さを測定し、当該測定結果を予め求めておいた破面遷移温度線図にプロットすることで、脆化度を判定するというものである。 In this method for detecting aging embrittlement, an electrolytic solution is brought into contact with a portion to be measured on the surface of a target metal through an opening of an electrolytic cell, a potential is applied to the portion to be measured, and the potential is swept in an increasing direction at 5 mV / sec. While measuring the current density flowing between the part to be measured and the counter electrode, and when this current density reaches a minimum value in the passive region, the sweeping of the potential is temporarily stopped, so that the passive film is applied to the part to be measured. After forming the passive film, the potential is similarly reversely swept at 5 mV / sec to confirm the minimum value of the current density of the electrolyte, and by holding the potential that gives the minimum value of the current density for a certain period of time, By selectively corroding the grain boundary part of the part to be measured, after measuring the depth of the grain boundary corrosion groove, and plotting the measurement result on the fracture surface transition temperature diagram obtained in advance, It is to determine the degree of conversion .

そして、このような判定方法によれば、被計測部に加えられる電位は不純物の偏析した粒界以外の領域が電解液によって不動態域になり、このため不動態皮膜の影響で腐食はほとんど起らなくなる。これ故、被計測部は、粒界部が腐食された部位と腐食されていない部位とに選択的にわかれるから、不純物の偏析により形成された粒界腐食溝の幅や深さ等を精度良く計測することができるようになり、脆化測定の信頼度が格段に高まるとしている。 According to such a determination method, the potential applied to the measurement target portion becomes a passive region by the electrolyte other than the grain boundary where the impurities are segregated, and therefore, corrosion hardly occurs due to the influence of the passive film. It will disappear. Therefore, since the part to be measured is selectively divided into a part where the grain boundary part is corroded and a part where it is not corroded, the width, depth, etc. of the grain boundary corrosion groove formed by segregation of impurities can be accurately determined. It becomes possible to measure, and the reliability of the embrittlement measurement is said to be remarkably increased.

尚、図３にアノード分極曲線のグラフを示してあるが、本明細書中においては、同図に示すように、自然電位(1)から不動態皮膜の生成が始まる最大電流密度発生電位(2)までの範囲を活性態域(a)と定め、この最大電流密度発生電位(2)から不動態皮膜の生成が終了して破壊が始まる電位(6)までを不動態域(b)と定め、この不動態皮膜の生成が終了して破壊が始まる電位(6)を超えた範囲を過不動態域(c)と定めている。また、上記最大電流密度発生電位(2)を不動態化電位と定め、上記不動態域（b）における最大電流密度発生電位(2)から２次アノードピーク電位(4)までの間の最小電流密度発生電位(3)を不動態化最小電位と定めている。更に、図２の往復分極曲線にて示してあるように、逆掃引時の折り返し点(R)から不動態化電位(2)までを再不動態域とし、この再不動態域における最小電流密度発生電位(8)を再不動態化最小電位と定めている。ここで、図３に示される不動態域(b)における不動態化電位(2)から不動態化最小電位(3)までの間は不動態皮膜の生成と腐食とが併存して同時進行する領域である。また、図２に示される逆掃引時の再不動態化最小電位(8)から不動態化電位(2)までの間は不動態皮膜の破壊と腐食とが併存して同時進行する領域である。
特開平５−２２３７２６号公報 FIG. 3 shows a graph of the anodic polarization curve. In this specification, as shown in FIG. 3, the maximum current density generation potential (2 ) Is defined as the active state region (a), and from this maximum current density generation potential (2) to the potential (6) where the formation of the passive film ends and the breakdown starts (6) is defined as the passive region (b). The range exceeding the potential (6) at which the formation of the passive film ends and the breakdown starts is defined as the overpassive region (c). The maximum current density generation potential (2) is defined as the passivation potential, and the minimum current between the maximum current density generation potential (2) and the secondary anode peak potential (4) in the passivation region (b). The density generation potential (3) is defined as the minimum passivation potential. Furthermore, as shown by the reciprocal polarization curve in FIG. 2, the repassive zone is defined as the repassive zone from the turnaround point (R) during reverse sweep to the passivating potential (2), and the minimum current density generation potential in this repassive zone. (8) is defined as the minimum repassivation potential. Here, during the period from the passivating potential (2) to the passivated minimum potential (3) in the passivated region (b) shown in FIG. It is an area. In addition, the region between the minimum repassivation potential (8) and the passivation potential (2) at the time of reverse sweep shown in FIG. 2 is a region in which destruction and corrosion of the passive film coexist and proceed simultaneously.
JP-A-5-223726

しかしながら、上記従来の方法では、被計測部の粒界部を選択的に腐食せしめるに際して、強い不動態皮膜を形成する金属材料（例えば、Cr−Mo−V鋼など）に対しては、粒界の選択的な腐食を十分に、かつ確実におこなわせることが難しく、このため粒界の腐食体積や面積を精度良く測定することは困難であった。
即ち、Cr−Mo−V鋼は強い不動態皮膜を形成することで知られているが、上記従来の粒界部の腐食方法では、逆掃引時において効果的に粒界溝のみを腐食することにならず、被腐食部位の全体（粒界表面を含む計測部全体）にも不動態皮膜が形成されてしまうことが、カソード電流が流れていることから容易に推定される。 However, in the conventional method described above, when the grain boundary portion of the measured portion is selectively corroded, the grain boundary is not applied to a metal material (for example, Cr-Mo-V steel) that forms a strong passive film. Therefore, it is difficult to sufficiently and reliably perform the selective corrosion, and it is difficult to accurately measure the corrosion volume and area of the grain boundary.
In other words, Cr-Mo-V steel is known to form a strong passive film, but the conventional grain boundary corrosion method effectively corrodes only the grain boundary grooves during reverse sweep. In addition, it is easily estimated that a passive film is formed on the entire corroded site (the entire measurement unit including the grain boundary surface) because the cathode current flows.

その結果として、不純物が偏析した部位の粒界の不動態皮膜を破壊して、かつ粒界溝を腐食させるためには、逆掃引時の電位を再不動態域における最小電流密度（極小電流密度）の発生電位（再不動態化最小電位）にて、かなりの時間（例えば20分程）保持する必要が生じることになるが、当該最小電流密度の発生電位にて逆掃引を一時的に止めて保持することは非常に困難である。即ち、電位の逆掃引中に最小電流密度の発生電位である再不動態化最小電位を事前に予測することは極めて難しい。しかも、最小電流密度の発生電位である再不動態化最小電位を一旦通過させてしまうと、不動態皮膜の破壊と腐食とが併存して同時進行する活性作用を生ずる領域に入ってしまい、表面状態は元には戻せなくなる。従って、少なくとも当該最小電流密度の発生電位まで下げる直前で逆掃引を停止させて保持せねばならない。しかしながら、当該最小電流密度の発生電位（再不動態化最小電位）の直前で逆掃引を停止させるのは非常に困難なことであって、確実性が低く、粒界溝の腐食形成の効率も劣るものであった。 As a result, in order to destroy the passive film at the grain boundary where the impurities segregate and corrode the grain boundary groove, the potential at the time of reverse sweep is set to the minimum current density in the repassive region (minimum current density). It is necessary to hold for a considerable time (for example, about 20 minutes) at the generation potential (repassivation minimum potential), but the reverse sweep is temporarily stopped and held at the generation potential of the minimum current density. It is very difficult to do. That is, it is extremely difficult to predict in advance the minimum repassivation potential, which is the generation potential of the minimum current density during the potential reverse sweep. In addition, once the repassivation minimum potential, which is the generation potential of the minimum current density, is allowed to pass, it enters the region where the passive film breaks down and corrodes simultaneously and causes an active action that proceeds simultaneously, and the surface state Cannot be restored. Therefore, the reverse sweep must be stopped and held at least immediately before the potential is lowered to the generation potential of the minimum current density. However, it is very difficult to stop the reverse sweep immediately before the generation potential (repassivation minimum potential) of the minimum current density, and the reliability is low and the efficiency of corrosion formation of the grain boundary groove is also inferior. It was a thing.

また、自然電位からの初期の掃引中には粒界のみならず、当該粒界を含む結晶粒全体が腐食されてしまう。そして、この様に結晶粒全体に腐食が生じると、各結晶間の結晶軸の方向性の相違に起因した腐食度合いの差が生じ、結果として結晶粒表面の各結晶間に高さ方向の段差が生じてしまうことになる。従って、粒界腐食溝の深さや幅、体積、面積等の測定値に基づいて金属の脆化評価を行うに当たっては、これらの測定は粒内面を基準として計測することになるので、上記の様に各結晶間に段差があると上記各種測定値の計測精度を下げることになってしまい、その信頼性の点でも改善の必要があった。 Further, during the initial sweep from the natural potential, not only the grain boundaries but also the entire crystal grains including the grain boundaries are corroded. When the entire crystal grain is corroded in this way, there is a difference in the degree of corrosion due to the difference in the direction of the crystal axis between the crystals, resulting in a height step between the crystals on the crystal grain surface. Will occur. Therefore, when evaluating the embrittlement of metals based on the measured values of the depth, width, volume, area, etc. of grain boundary corrosion grooves, these measurements are made with reference to the grain inner surface. If there is a step between the crystals, the measurement accuracy of the above various measured values is lowered, and it is necessary to improve the reliability.

本発明は、以上のような従来の課題に鑑みて創案されたものであり、その目的の一つは、結晶粒表面の過剰な金属腐食（金属溶解）を可及的に抑えつつ、当該結晶粒表面に確実にムラなく不動態皮膜を形成できるとともに、各結晶相互間の表面高さに段差が生じることを可及的に小さく抑えることができる金属表面の腐食方法を提供することにある。また、他の目的は、最小電流密度の発生電位にて逆掃引を停止させて保持することなく、自然電位まで戻す間で容易にかつ確実に、しかも効率良く、不純物が偏析した粒界部位の不動態皮膜を選択的に破壊して当該部位の金属腐食（金属溶解）を促進させることができる金属表面の腐食方法を提供することにある。 The present invention was devised in view of the conventional problems as described above, and one of its purposes is to suppress the excessive metal corrosion (metal dissolution) on the crystal grain surface as much as possible. It is an object of the present invention to provide a method for corroding a metal surface that can form a passive film on the grain surface without any unevenness and can suppress the occurrence of a step in the surface height between the crystals as small as possible. In addition, another object is to easily and surely and efficiently reduce the grain boundary site where impurities are segregated while returning to the natural potential without stopping and holding the reverse sweep at the generation potential of the minimum current density. An object of the present invention is to provide a method for corroding a metal surface, which can selectively destroy a passive film and promote metal corrosion (metal dissolution) at the site.

上記課題を解決するために、本願の請求項１に係る発明においては、金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、前記自然電位から不動態化電位までの活性態域では、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引する、ことを特徴とする。 In order to solve the above problems, in the invention according to claim 1 of the present application, a potential is applied by bringing the corrosion target portion of the metal surface into contact with the electrolytic solution, and the potential is passed from the natural potential to the active state region. Sweep in the upward direction to an arbitrary value exceeding the mobilization potential to form a passive film on the corrosion target site, and then reversely sweep the potential in the downward direction to pass the repassivation and reactivation states. The metal surface grain boundary corrosion method selectively corrodes the crystal grain boundary portion of the metal surface by lowering to the natural potential, and in the active state region from the natural potential to the passivation potential, It is characterized by sweeping at a high speed so as to activate the part to be corroded while suppressing the corrosion of the whole grain.

本願の請求項２に係る発明においては、金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、前記逆掃引時には少なくとも前記再不動態域の再不動態化最小電位までは、粒界部の溶解を促進すべく遅い速度で掃引する、ことを特徴とする。 In the invention according to claim 2 of the present application, a potential is applied by bringing the corrosion target portion of the metal surface into contact with the electrolytic solution, and the potential is passed from the natural potential to the active state region and exceeds the passivation potential. Swept up to an upward direction to form a passive film at the site to be corroded, and then reversely swept the electric potential in the downward direction to pass through the repassivation zone and the reactivation zone to lower it to the natural potential. Is a method for selectively corroding a crystal grain boundary portion of a metal surface, wherein the grain boundary portion is dissolved at least up to the repassivation minimum potential in the repassivation region during the reverse sweep. It is characterized by sweeping at a slow speed to promote.

本願の請求項３に係る発明においては、金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、前記不動態域の不動態化電位から自然電位に戻すまでは、粒界部の溶解を促進すべく遅い速度で掃引する、ことを特徴とする。 In the invention according to claim 3 of the present application, an electric potential is applied by bringing the corrosion target portion of the metal surface into contact with the electrolytic solution, and the electric potential is passed from the natural potential to the active state region and exceeds the passivation potential. Swept up to an upward direction to form a passive film at the site to be corroded, and then reversely swept the electric potential in the downward direction to pass through the repassivation zone and the reactivation zone to lower it to the natural potential. This is a method for intergranular corrosion on the metal surface that selectively corrodes the crystal grain boundary part on the metal surface, and promotes dissolution of the grain boundary part until the passivated potential in the passive region is returned to the natural potential. The sweeping is as slow as possible.

本願の請求項４に係る発明においては、金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、前記自然電位から不動態化電位までの活性態域では、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引し、前記不動態化電位から自然電位に戻すまでは粒界部の溶解を促進すべく遅い速度で掃引する、ことを特徴とする。 In the invention according to claim 4 of the present application, an electric potential is applied by bringing the corrosion target portion of the metal surface into contact with the electrolytic solution, and the electric potential is passed from the natural potential to the active state region and exceeds the passivation potential. Swept up to an upward direction to form a passive film at the site to be corroded, and then reversely swept the electric potential in the downward direction to pass through the repassivation zone and the reactivation zone to lower it to the natural potential. Is a method of intergranular corrosion on the metal surface that selectively corrodes the crystal grain boundary portion of the metal surface, and in the active state region from the natural potential to the passivation potential, while suppressing the corrosion of the entire crystal grain It is characterized by sweeping at a high speed to activate the site to be corroded and sweeping at a low speed to promote dissolution of the grain boundary portion until returning from the passivation potential to the natural potential.

ここで、請求項５に示すように、上記請求項１〜４において、前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点は、不動態化電位での電流密度に等しい電流密度を発生させる電位（図３の(7)に示す電位）以下の任意値となし得る。 Here, as shown in claim 5, in the above claims 1 to 4, the turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction is a current density equal to the current density at the passivation potential. Can be an arbitrary value below the potential (the potential shown in (7) of FIG. 3).

或いは、請求項６に示すように、上記請求項１〜４において、前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点は、過不動態化電位以下の任意値となし得る。 Alternatively, as shown in claim 6, in the above claims 1 to 4, the turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction can be an arbitrary value equal to or less than the overpassivation potential.

或いは、請求項７に示すように、上記請求項１〜４において、前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点は、不動態化最小電位に至る前の近傍の電位から過不動態化電位以下の範囲の任意値となし得る。 Alternatively, as shown in claim 7, in the above claims 1 to 4, the turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction is from a potential in the vicinity before reaching the minimum passivation potential. It can be any value in the range below the hyperpassivation potential.

或いは、請求項８に示すように、上記請求項１〜４において、前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点は、不動態化最小電位以上で過不動態化電位以下の範囲の任意値となし得る。 Alternatively, as shown in claim 8, in the above claims 1 to 4, the turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction is not less than the minimum passivation potential and not more than the overpassivation potential. Can be any value in the range.

また、請求項９に示すように、上記請求項４〜８において、前記自然電位から不動態化電位までの活性態域では、その掃引速度を１０mV／sec 〜１００mV／sec とし、前記不動態化電位から前記再不動態域の再不動態化最小電位までは、その掃引速度を０．１mV／sec 〜２mV／secとなすのが望ましい。 Further, as shown in claim 9, in the above-mentioned claims 4 to 8, in the active state region from the natural potential to the passivation potential, the sweep rate is 10 mV / sec to 100 mV / sec, and the passivation is performed. From the potential to the minimum repassivation potential in the repassivation region, the sweep rate is preferably 0.1 mV / sec to 2 mV / sec.

また、上記請求項１〜９において、前記金属がＣｒ−Ｍｏ−Ｖ鋼等の低合金鋼である構成となし得る。 In the first to ninth aspects, the metal may be a low alloy steel such as Cr-Mo-V steel.

本願発明の請求項１に係る金属表面の粒界腐食方法に示すように、金属表面の腐食対象部位に加える電位を掃引するにあたって、自然電位から不動態皮膜の生成が始まる不動態域の不動態化電位（最大電流密度発生電位）までの活性態域の掃引速度を、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引すれば、結晶粒表面の過剰な金属腐食（金属溶解）を可及的に抑えつつ、当該結晶粒表面に確実にムラなく不動態皮膜を形成でき、もって結晶軸方向がランダムで腐食の進行度合いが異なっている結晶粒相互間にあっても、その表面高さに段差が生じることを可及的に小さく抑えることができる。 As shown in the intergranular corrosion method for a metal surface according to claim 1 of the present invention, when the potential applied to the corrosion target site on the metal surface is swept, the passivation in the passive region where the generation of the passive film starts from the natural potential. If the sweep speed of the active state up to the crystallization potential (maximum current density generation potential) is swept at a high speed to activate the corrosion target site while suppressing corrosion of the entire crystal grain, excess metal on the crystal grain surface While suppressing corrosion (metal dissolution) as much as possible, it is possible to reliably form a passive film on the surface of the crystal grains, even between crystal grains with random crystal axis directions and different degrees of corrosion progression. It is possible to suppress the occurrence of a step in the surface height as much as possible.

請求項２に係る金属表面の粒界腐食方法に示すように、金属表面の腐食対象部位に加える電位を、不動態化電位を超えた任意値まで掃引してから自然電位まで逆掃引して戻すにあたって、前記逆掃引時には少なくとも再不動態域の最小電流密度の発生電位である再不動態化最小電位までは、粒界部の溶解を促進すべく遅い速度で掃引すれば、当該最小電流密度の発生電位にて掃引を停止させて保持することなく、結晶粒面に不動態皮膜を形成して、かつ燐等の脆化元素や不純物が偏析する粒界の不動態皮膜は極薄く形成できる。このため、爾後の自然電位まで戻す間で、容易にかつ確実に、しかも効率良く、不純物が偏析した粒界部位の不動態皮膜を選択的に破壊して当該部位の金属腐食（金属溶解）を促進させることができるようになり、もって粒界溝の幅や深さを大きく形成できる。 As shown in the intergranular corrosion method of the metal surface according to claim 2, the potential applied to the corrosion target portion of the metal surface is swept up to an arbitrary value exceeding the passivation potential and then reversely swept back to the natural potential. At the time of the reverse sweep, at least to the repassivation minimum potential that is the generation potential of the minimum current density in the repassivation region, if the sweep is performed at a slow speed to promote the dissolution of the grain boundary part, the generation potential of the minimum current density is reached. Without stopping the sweeping and holding, a passive film is formed on the crystal grain surface, and a passive film at the grain boundary where segregation of embrittlement elements and impurities such as phosphorus segregates can be formed extremely thin. For this reason, while returning to the natural potential after dripping, the passive film of the grain boundary part where the impurities segregated can be selectively destroyed easily and reliably, and the metal corrosion (metal dissolution) of the part can be prevented. Thus, the width and depth of the grain boundary groove can be increased.

請求項３に係る金属表面の粒界腐食方法に示すように、不動態皮膜の生成が始まる不動態化電位から当該不動態域内における不動態化最小電位以上の任意値まで電位を上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を自然電位まで逆掃引する際の掃引速度を、粒界部の溶解を促進すべく遅い速度で掃引すれば、当該再不動態域における最小電流密度発生電位である再不動態化最小電位にて、粒界部の溶解促進を目的としてその逆掃引を停止させて保持することなく、結晶粒面に不動態皮膜を形成して、かつ燐等の脆化元素や不純物が偏析する粒界の不動態皮膜は破壊することができる。このため、再不動態化最小電位から自然電位まで戻す間で、容易にかつ確実に、しかも効率良く、不純物が偏析した粒界部位の不動態皮膜を選択的に破壊して当該部位の金属腐食（金属溶解）を促進させることができ、もって粒界溝の幅や深さを大きく形成できるようになる。 As shown in the intergranular corrosion method of the metal surface according to claim 3, the potential is swept in the upward direction from the passivation potential at which the formation of the passive film starts to any value above the minimum passivation potential in the passive region. Then, after forming a passive film on the site to be corroded, if the sweep speed when the potential is swept back to the natural potential is swept at a slow speed to promote dissolution of the grain boundary portion, the repassivation region Forming a passive film on the crystal grain surface without stopping and holding the reverse sweep for the purpose of promoting dissolution of the grain boundary at the minimum re-passivation potential that is the minimum current density generation potential in Passive films at grain boundaries where segregation of embrittlement elements and impurities such as phosphorus segregate can be destroyed. For this reason, while returning from the minimum repassivation potential to the natural potential, it easily and reliably and efficiently destroys the passive film at the grain boundary site where the impurities segregate, and the metal corrosion ( (Metal dissolution) can be promoted, so that the width and depth of the grain boundary groove can be increased.

請求項４に係る金属表面の粒界腐食方法によれば、金属表面の腐食対象部位に加える電位を掃引するにあたって、自然電位から不動態皮膜の生成が始まる不動態化電位までの活性態域の掃引速度を、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引する一方、不動態皮膜の生成が始まる当該不動態化電位からこれを超えた任意値まで上昇方向に掃引して腐食対象部位に不動態皮膜を形成してから、該電位の掃引を下降方向に折り返して自然電位まで逆掃引する際の掃引速度を、粒界部の溶解を促進すべく遅い速度で掃引するので、結晶粒表面の過剰な金属腐食（金属溶解）を可及的に抑えつつ、当該結晶粒表面に確実にムラなく不動態皮膜を形成でき、もって結晶軸方向がランダムで腐食の進行度合いが異なっている結晶粒相互間にあっても、その表面高さに段差が生じることを可及的に小さく抑えることができる。さらに、当該最小電流密度の発生電位にて掃引を停止させて保持することなく、結晶粒面に不動態皮膜を形成して、かつ燐等の脆化元素や不純物が偏析する粒界の不動態皮膜は破壊することができる。このため、自然電位まで戻す間で、容易にかつ確実に、しかも効率良く、不純物が偏析した粒界部位の不動態皮膜を選択的に破壊して当該部位の金属腐食（金属溶解）を促進させることができ、もって粒界溝の幅や深さを大きく形成でき、当該粒界溝の計測を容易に、かつ高精度に行わせることができるようになる。 According to the intergranular corrosion method of the metal surface according to claim 4, in sweeping the potential applied to the corrosion target site on the metal surface, the active state region from the natural potential to the passivation potential where the formation of the passive film starts. While sweeping the sweep rate at a fast rate to activate the target corrosion site while suppressing the corrosion of the entire crystal grain, it increases from the passivating potential at which the passivated film starts to an arbitrary value beyond it. After forming a passive film at the site to be corroded to reverse the sweep of the potential in the downward direction and back sweeping to the natural potential, the slow speed to promote dissolution of the grain boundary part Therefore, while suppressing excessive metal corrosion (metal dissolution) on the crystal grain surface as much as possible, a passive film can be reliably formed on the crystal grain surface, and the crystal axis direction is random and corrosive. Different progress Be located between the crystal grains mutually have, it can be kept as small as possible that the step is formed on the surface height. Furthermore, without stopping and maintaining the sweep at the generation potential of the minimum current density, a passive film is formed on the crystal grain surface, and the passivation of the grain boundary where segregation of embrittlement elements and impurities such as phosphorus segregates. The film can be destroyed. For this reason, while returning to the natural potential, the passive film at the grain boundary part where the impurities are segregated is easily and reliably and efficiently destroyed to promote metal corrosion (metal dissolution) at the part. Accordingly, the width and depth of the grain boundary groove can be formed large, and the measurement of the grain boundary groove can be performed easily and with high accuracy.

以下に、本発明に係る金属表面の粒界腐食方法の好適な実施の形態について、添付図面を参照して詳述する。 Hereinafter, preferred embodiments of a method for intergranular corrosion of a metal surface according to the present invention will be described in detail with reference to the accompanying drawings.

本発明は金属表面の粒界を腐食させるにあたって、金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を図３のアノード分極曲線のグラフに示すように、自然電位(1)から活性態域(a)を通過させて不動態域(b)における不動態化最小電位(3)を超えた任意値に至るまで上昇方向に掃引して腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に折り返して逆掃引し、再び不動態域(b）と活性態域(a)とを通過させて自然電位(1)まで戻して往復掃引することで、金属表面の結晶粒界部を選択的に腐食させるという、電気化学的再活性化法（ＥＰＲ法）を利用するものである。 In the present invention, when the grain boundary on the metal surface is corroded, the corrosion target portion on the metal surface is brought into contact with the electrolytic solution, and a potential is applied. As shown in the graph of the anodic polarization curve in FIG. ) From the active state region (a) to the arbitrary value exceeding the minimum passivating potential (3) in the passive region (b) to form a passive film at the corrosion target site. After that, the potential is turned back in the downward direction and reversely swept, and again through the passive region (b) and the active state region (a) to return to the natural potential (1) and swept back and forth to obtain a metal surface. An electrochemical reactivation method (EPR method) that selectively corrodes the crystal grain boundary is used.

ここで、上記折り返し点としては、不動態域の始まる不動態化電位(2)を超えた任意値で良いのであるが、その上限は上記不動態化電位(2)にて発生する電流密度を超えさせないようにして、過不動態域(c)において不動態化電位(2)での電流密度に等しい電流密度を発生させる電位(7)以下にするのが好ましく、より好ましくは過不動態域(c)には達しないように過不動態化電位（6）以下とするのが良い。更に好ましくは、不動態化最小電位(3)に至る前の近傍の電位から過不動態化電位(6)に至るまでの範囲に設定するのが良く、最も望ましくは、不動態化最小電位(3)を超えた直後を折り返し点とするのが最良である。 Here, the turning point may be an arbitrary value exceeding the passivation potential (2) at which the passive region starts, but the upper limit is the current density generated at the passivation potential (2). It is preferable not to exceed the potential (7) or less, more preferably the potential of the passive state (c) to generate a current density equal to the current density at the passivating potential (2). It is good to set it to the overpassivation potential (6) or less so as not to reach (c). More preferably, it should be set in the range from the potential in the vicinity before reaching the minimum passivation potential (3) to the hyperpassivation potential (6), and most preferably the minimum passivation potential ( It is best to set the turning point immediately after exceeding 3).

そこで、本実施の形態では、図２の往復分極曲線、および図３のアノード分極曲線のグラフに示すように、当該不動態化最小電位(3)を超えた直後の電位を折り返し点(R)としている。そして、本願発明では、当該掃引する電位を折り返して往復掃引するに際して、前記自然電位(1)から不動態域(b)の最大電流密度発生電位である不動態化電位(2)までの上昇側の活性態域（a）では、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引すること、および当該不動態域(b)の不動態化電位(2)を超えた任意値の折り返し点(R)まで電位を上昇掃引させた後、当該折り返し点(R)で折り返して再び不動態域(b)の再不動態化最小電位（最小電流密度発生電位）(8)を経て自然電位(1)まで電位を下降方向に逆掃引する間は、粒界部の溶解を促進すべく当該電位の掃引速度を遅くすること、或いは少なくとも折り返し点(R)からの逆掃引時において再不動態化最小電位(8)に至る迄の間はその逆掃引速度を遅くすること、を特徴的な事項とするものである。 Therefore, in the present embodiment, as shown in the graphs of the reciprocal polarization curve in FIG. 2 and the anodic polarization curve in FIG. 3, the potential immediately after exceeding the passivation minimum potential (3) is the turning point (R). It is said. In the present invention, when the potential to be swept is turned back and forth, the rising side from the natural potential (1) to the passivation potential (2) which is the maximum current density generation potential in the passive region (b). In the active state region (a), sweeping at a high speed to activate the corrosion target site while suppressing the corrosion of the entire crystal grains, and the passivation potential (2) of the passive region (b) After the potential is swept up to the turn-around point (R) of an arbitrary value that has exceeded, it turns back at the turn-back point (R), and then repassivation minimum potential (minimum current density generation potential) in the passive region (b) (8 ) To the natural potential (1) in the downward direction, the potential sweep rate is slowed to promote dissolution of the grain boundary, or at least the reverse sweep from the turning point (R). The reverse sweep speed is slowed down until the minimum repassivation potential (8) is reached. It is to be a matter of interest.

即ち、図１は電気化学的な再活性化法（ＥＰＲ法）で金属表面の粒界を腐食させる場合に用いられている従来からよく知られた装置の概略構成を示す図であり、本発明においてもこの装置を使用する。この装置２は内部に電解液４を保持するセル６と、このセル６の開口部が密着されて取り付けられて電解液４に接触させられる腐食対象部位としての試験電極８と、この試験電極８に適正な電位を付与するための照合電極１０と、白金でなる対極１２と、電位の掃引を制御しかつ分極曲線を記録するためのパソコンを含むポテンショスタット１４とからなり、試験電極８に流れる電流密度を監視しつつ当該試験電極８に加える電位を任意に制御し得るようになっている。つまり、電流密度を監視しながら当該電位を自然電位(1)から増大させて上昇方向に掃引し、不動態化電位(2)を超えた所望の任意値の折り返し点(R)に達した時点で逆に電位を減少させて下降方向に逆掃引することによって、不動態皮膜の形成を制御し得るようになっている。 That is, FIG. 1 is a diagram showing a schematic configuration of a conventionally well-known apparatus used when a grain boundary on a metal surface is corroded by an electrochemical reactivation method (EPR method). This device is also used in The apparatus 2 includes a cell 6 that holds an electrolyte solution 4 therein, a test electrode 8 that serves as a corrosion target portion that is attached with the opening of the cell 6 being brought into close contact with the electrolyte solution 4, and the test electrode 8. And a potentiostat 14 including a personal computer for controlling a potential sweep and recording a polarization curve, and flows to the test electrode 8. The potential applied to the test electrode 8 can be arbitrarily controlled while monitoring the current density. In other words, while monitoring the current density, the potential is increased from the natural potential (1), swept in the upward direction, and reached a desired arbitrary turning point (R) exceeding the passivation potential (2). On the contrary, the formation of the passive film can be controlled by decreasing the potential and performing reverse sweeping in the downward direction.

ここで、本実施の形態では、腐食させる対象金属は発電用のタービンに用いられている低合金鋼のＣｒ−Ｍｏ−Ｖ鋼として、その脆化度を非破壊試験で判定のために燐等の不純物が偏析した部分の粒界部を選択的に腐食させる場合を例示する。 Here, in the present embodiment, the target metal to be corroded is Cr-Mo-V steel, which is a low alloy steel used in a turbine for power generation, and the degree of embrittlement is determined by phosphorus or the like for determination in a nondestructive test. The case where the grain boundary part of the part where the impurities of segregation are selectively corroded is illustrated.

先ず、脆化を評価する鋼材の検査部位（腐食対象部位）であって、試験電極８となる金属表面をクリーニングして付着しているスケール等を除去した後、当該試験電極８を研磨剤で鏡面に仕上げる。その後、電解セル６を試験電極８に貼り付けて、当該試験電極８と電解液４との試験温度を確認してから、ポテンショスタット１４を電解セル６に繋ぐ。そして、電解液（ピクリン酸飽和水溶液、あるいは飽和ピクリン酸に酸化性を高めるために硝酸等を0．5％以下の微量添加した水溶液〉を介して試験電極８に加える電位を、電流密度を監視しながら制御する。この電位の制御は、図２のグラフに示すように、その自然電位（1）から不動態域(b)の最大電流密度発生電位である不動態化電位(2)に至るまでは、比較的早い掃引速度（具体的には１０mV／sec 〜１００mV／sec）で電位の増大する上昇方向に掃引し、結晶粒全体の腐食を抑制しながら検査部位の試験電極８を活性化させる。 First, after removing the scale etc. which are the test | inspection site | part (corrosion object site | part) of the steel material which evaluates embrittlement, the metal surface used as the test electrode 8 adheres, the said test electrode 8 is made with an abrasive | polishing agent. Finish the mirror. Thereafter, the electrolytic cell 6 is attached to the test electrode 8 and the test temperature of the test electrode 8 and the electrolytic solution 4 is confirmed, and then the potentiostat 14 is connected to the electrolytic cell 6. The potential applied to the test electrode 8 through the electrolytic solution (a saturated aqueous solution of picric acid or an aqueous solution containing nitric acid or the like added in a small amount of 0.5% or less in order to increase the oxidizing property of the saturated picric acid) is monitored for the current density. As shown in the graph of FIG. 2, this potential is controlled from the natural potential (1) to the passivation potential (2) which is the maximum current density generation potential in the passive region (b). Until the test electrode 8 at the inspection site is activated while sweeping in the direction of increasing potential at a relatively fast sweep speed (specifically, 10 mV / sec to 100 mV / sec) and suppressing corrosion of the entire crystal grains. Let

次に、不動態域(b)における不動態化最小電位(3)以上で過不動態化電位(6）以下の所望の任意値の折り返し点(R)にまで電位が到達したならば、今度はその電位を減少方向に逆掃引する。ここで、本実施の形態では、当該逆掃引への折り返し点(R)となる上記任意値を不動態化最小電位(3)に設定しているが、当該任意値は２次アノードピーク電位(4)、不動態化中央電位(5)等に設定しも良い。或いは、不動態化最小電位（3）に至る前の近傍に設定しても良いし、不動態化電位(2)で発生する電流密度に等しい電流密度が発生する過不動態域(c)の電位（7）に設定しても良い。また、この逆掃引をするにあたって、上記折り返し点(R)への到達時における不動態皮膜形成のための保持時間は、２分以下と極短くする。あるいは全く保持時間を持たせずに直ぐに逆掃引に入るようにしても良い。 Next, if the potential reaches the desired arbitrary turn-back point (R) above the minimum passivating potential (3) and below the passivating potential (6) in the passivating zone (b), Reverse sweeps the potential in the decreasing direction. Here, in the present embodiment, the above-mentioned arbitrary value that becomes the turning point (R) to the reverse sweep is set to the minimum passivating potential (3), but the arbitrary value is the secondary anode peak potential ( 4), passivated median potential (5), etc. may be set. Alternatively, it may be set in the vicinity before reaching the minimum passivating potential (3), or in the overpassive zone (c) where a current density equal to that generated at the passivating potential (2) is generated. It may be set to potential (7). Further, when performing this reverse sweep, the holding time for forming the passive film when reaching the turn-back point (R) is extremely short, such as 2 minutes or less. Alternatively, the reverse sweep may be started immediately without having any holding time.

一方、上記不動態化電位（2）から逆掃引後の再不動態域における最小電流密度の発生電位である再不動態化最小電位（8）に至るまでの間は、比較的遅い掃引速度（具体的には0．1mV／sec〜2mV／sec）で電位の増大方向に掃引および電位の減少方向に逆掃引して、不動態皮膜を形成する。このとき、粒界部の不純物が偏析した部位の不動態皮膜は不安定で破壊され易いものとなる。 On the other hand, during the period from the above passivation potential (2) to the repassivation minimum potential (8), which is the generation potential of the minimum current density in the repassivation region after reverse sweep, a relatively slow sweep rate (specifically In the case of 0.1 mV / sec to 2 mV / sec), the film is swept in the direction of increasing potential and reversely swept in the direction of decreasing potential to form a passive film. At this time, the passive film at the site where the impurities at the grain boundary part are segregated becomes unstable and easily broken.

また、再不動態化最小電位（8）に到達した後は、引き続き逆掃引を行って、粒界のみの選択的腐食を行う。この場合の掃引速度は同じく0．1mV／sec〜2mV／secと比較的遅い速度で掃引を行い、粒界のみの腐食を効果的に行う。
即ち、再不動態化最小電位（8）に至った後の逆掃引時において、粒界部の不純物偏析部位に形成された不安定な不動態皮膜を破壊させて粒界部の腐食を選択的に行わせる。 In addition, after reaching the repassivation minimum potential (8), the reverse sweep is continued to perform selective corrosion of only the grain boundaries. In this case, the sweep rate is similarly 0.1 mV / sec to 2 mV / sec, and the sweep is performed at a relatively slow rate, and only the grain boundary is effectively corroded.
That is, at the time of reverse sweep after reaching the minimum repassivation potential (8), the unstable passivation film formed at the impurity segregation site in the grain boundary part is destroyed to selectively corrode the grain boundary part. Let it be done.

そして、上記の本発明による粒界腐食方法で粒界腐食を行った後、検査部位８に対してシート状フィルム（アセチルセルロース）をこれに溶剤（酢酸メチル）を滴下して貼り付けて、結晶粒界を転写し、レーザ顕微鏡にて焦点移動メモリ画像（ＦＳＭ画像）や白黒濃淡画像（Ｚ画像）などの二次元表面形状画像を得て、粒界腐食溝の最大深さと幅および結晶粒表面の粗さを測定する。 And after performing intergranular corrosion by the above-mentioned intergranular corrosion method according to the present invention, a sheet-like film (acetylcellulose) is dropped onto the inspection site 8 and a solvent (methyl acetate) is dropped onto the crystal to be crystallized. Grain boundaries are transferred and two-dimensional surface shape images such as focus movement memory images (FSM images) and black-and-white grayscale images (Z images) are obtained with a laser microscope to obtain the maximum depth and width of grain boundary corrosion grooves and the grain surface. Measure the roughness.

この時、粒界溝に隣接する２つの金属結晶粒の表面に高さ方向の段差があった場合には、それら２つの金属結晶粒表面にエッヂ部同士を繋いだ傾斜した面までを粒界溝部分とみなして、表面高さの低い金属結晶粒の表面を延長して区画される粒界溝の体積に、さらにその上方にある上記傾斜面に至る部分の体積に可及的に近似した体積値を加算して補正する処理をしている。 At this time, if there is a step in the height direction on the surface of the two metal crystal grains adjacent to the grain boundary groove, the grain boundary extends to the inclined surface connecting the edge portions to the two metal crystal grain surfaces. Considering it as a groove part, the volume of the grain boundary groove that is defined by extending the surface of the metal crystal grain with a low surface height is approximated as much as possible to the volume of the part that reaches the inclined surface above it. The volume value is added and corrected.

また、上記最大の粒界幅より少し大きい幅で、二次元表面形状画像の焦点移動メモリ画像（ＦＳＭ画像）の粒界部をトレースした画像（マスク画像）を作成し、このマスク画像で覆われた粒界溝部分の三次元画像データから、上記粒界溝体積（粒界腐食体積）の他に、粒界溝断面積（粒界腐食面積）、粒界溝長さ及び粒界溝最大深さ（粒界腐食最大深さ）、粒界溝平均断面積（粒界腐食平均断面積）、粒界溝平均深さ（粒界腐食平均深さ）を測定または算出している。そして、金属部材の脆化度の評価をするにあたって、これらの測定値や算出値を基礎とし、この算出値等からさらに粒界腐食体積、粒界腐食面積、粒界長さ及び粒界腐食最大深さ、粒界腐食平均断面積、粒界腐食平均深さ等を算出して金属材料の脆化度を高精度に評価している。 Also, an image (mask image) obtained by tracing the grain boundary portion of the focus movement memory image (FSM image) of the two-dimensional surface shape image with a width slightly larger than the maximum grain boundary width is created and covered with this mask image. In addition to the above-mentioned grain boundary groove volume (intergranular corrosion volume), the grain boundary groove cross-sectional area (intergranular corrosion area), grain boundary groove length, and maximum grain boundary groove depth are obtained from the three-dimensional image data of the grain boundary groove part. (Intergranular corrosion maximum depth), intergranular groove average cross-sectional area (intergranular corrosion average cross-sectional area), and intergranular groove average depth (intergranular corrosion average depth) are measured or calculated. Then, when evaluating the degree of embrittlement of a metal member, based on these measured values and calculated values, the intergranular corrosion volume, intergranular corrosion area, intergranular length, and intergranular corrosion maximum are further calculated from these calculated values. Depth, intergranular corrosion average cross-sectional area, and intergranular corrosion average depth are calculated to evaluate the degree of embrittlement of metal materials with high accuracy.

図４と図５と図６は、上述した金属表面の粒界腐食方法においてその各種の条件を違えて金属表面を腐食させた場合の、再不動態化最小電流密度値Ｉｒ、表面粗さ、粒界溝体積の測定結果、および粒界溝体積と全腐食電流量との比（粒界溝体積/全腐食電流量）を対比したものである。但し、逆掃引への折り返し点（R）の保持電位は全ての試験片において、不動態化最小電位にしている。また、当該折り返し点（R）での保持時間は全て２ｍｉｎとしている。 4, 5, and 6 show the repassivation minimum current density value Ir, surface roughness, grain size, and the like when the metal surface is corroded under different conditions in the above-described intergranular corrosion method of the metal surface. This is a comparison of the measurement results of the inter-slot volume, and the ratio of the inter-granular groove volume to the total corrosion current amount (intergranular groove volume / total corrosion current amount). However, the holding potential at the turn-back point (R) to the reverse sweep is set to the minimum passivation potential in all the test pieces. In addition, the holding time at the turn-back point (R) is all 2 minutes.

そして、比較対象の基準とする基準試験片は往路分極速度（掃引速度）と復路分極速度（逆掃引速度）を共に等しく１ｍＶ／ｓとした。試験片Ｎｏ.１は往路分極速度（掃引速度）を１０ｍＶ／ｓ、復路分極速度（逆掃引速度）を１ｍＶ／ｓとした。試験片Ｎｏ.２は往路分極速度（掃引速度）を１０ｍＶ／ｓ、復路分極速度（逆掃引速度）を０．５ｍＶ／ｓとした。試験片Ｎｏ.３は往路分極速度（掃引速度）を１００ｍＶ／ｓ、復路分極速度（逆掃引速度）を０．１６７ｍＶ／ｓとした。 The reference test piece used as a reference for comparison was set to 1 mV / s for both the forward polarization speed (sweep speed) and the backward polarization speed (reverse sweep speed). Specimen No. 1 had a forward polarization speed (sweep speed) of 10 mV / s and a backward polarization speed (reverse sweep speed) of 1 mV / s. For test piece No. 2, the forward polarization speed (sweep speed) was 10 mV / s, and the backward polarization speed (reverse sweep speed) was 0.5 mV / s. For test piece No. 3, the forward polarization speed (sweep speed) was 100 mV / s, and the backward polarization speed (reverse sweep speed) was 0.167 mV / s.

ここで、結晶粒表面に生じる段差を少なくしつつ、結晶粒界を深く腐食させるという観点からすると、まず、腐食量は消費電力量に相当するから、再不動態化最小電位（8）時の電流密度値Ｉｒはその腐食量に大小に関与し、よって当該電流密度値Ｉｒは原則的には大きい方が良い。しかしながら、その腐食の発生部位が粒界部分に集中しているか否かを判断するには表面粗さと粒界溝体積とを合わせて考慮する必要がある。
即ち、表面粗さについては、結晶粒表面に段差が出来ていると平均粗さＲａは大きくなると予測し得るので、当該平均粗さＲａは小さい方が良い。また、最大深さＲｙは粒界部分が腐食されている筈なので、大きい方が良いと考えられる。また、１０点平均粗さＲｚは粒界部分と見なし得るから、大きい方が良いと考えられる。 Here, from the viewpoint of deeply corroding the grain boundary while reducing the level difference on the crystal grain surface, first, the amount of corrosion corresponds to the power consumption, so the current at the minimum repassivation potential (8) The density value Ir is related to the amount of corrosion, and therefore the current density value Ir is preferably larger in principle. However, it is necessary to consider both the surface roughness and the grain boundary groove volume in order to determine whether or not the corrosion occurrence site is concentrated at the grain boundary part.
That is, with respect to the surface roughness, it can be predicted that the average roughness Ra will increase if there is a step on the crystal grain surface. Therefore, it is preferable that the average roughness Ra is small. Moreover, since the maximum depth Ry should have corroded the grain boundary part, it is thought that the larger one is better. Further, since the 10-point average roughness Rz can be regarded as a grain boundary part, it is considered that a larger value is better.

また、全腐食電流量（消費電力量）に対して、形成された粒界溝の体積が大きければ大きい程、粒界部に集中的に効率よく腐食が生じていることになるので、当該粒界溝体積と全腐食電流量との比（粒界溝体積/全腐食電流量）は大きい方が良いと考えられる。 In addition, the larger the volume of the formed grain boundary groove with respect to the total amount of corrosion current (power consumption), the more concentrated and efficient corrosion occurs at the grain boundary part. It is considered that a larger ratio of the inter-slot volume and the total corrosion current amount (grain boundary groove volume / total corrosion current amount) is better.

さらに、粒界溝体積を考慮すると、当該粒界溝体積の補正割合が少ないことが、結晶粒表面の段差が小さいことになる。つまり、補正値である上述した傾斜部体積（表面高さの低い金属結晶粒の表面を延長して区画される粒界溝部分よりも上方にある傾斜面に至る部分の体積）と粒界溝体積との比（傾斜部体積／粒界溝体積）が小さい方が良いと考えられる。 Furthermore, when the grain boundary groove volume is taken into consideration, a small correction ratio of the grain boundary groove volume means that the step on the crystal grain surface is small. That is, the above-mentioned slope volume (correction value) (volume of the portion reaching the sloped surface above the grain boundary groove portion defined by extending the surface of the metal crystal grain having a low surface height) and the grain boundary groove It is considered that a smaller ratio of the volume (volume of the inclined portion / volume of the grain boundary groove) is better.

よってこれらの点を考慮すると、試験片Ｎｏ.１、Ｎｏ.２、Ｎｏ３は基準片に比して明らかに優れており、これは表１〜３に示される結晶粒表面の粗さと粒界溝体積の測定値、並びに傾斜部体積と粒界溝体積との比、粒界溝体積と全腐食電流量との比からも明らかに裏付けられている。 Therefore, in consideration of these points, the test pieces No. 1, No. 2, and No. 3 are clearly superior to the reference pieces, which are the grain surface roughness and grain boundary grooves shown in Tables 1 to 3. This is clearly supported by the measured value of the volume, the ratio of the inclined portion volume to the grain boundary groove volume, and the ratio of the grain boundary groove volume to the total corrosion current amount.

従って、以上に説明したように、本実施形態の金属表面の粒界腐食方法では、金属表面の腐食対象部位である試験電極８に加える電位を掃引するにあたって、自然電位（1）から不動態皮膜の生成が始まる不動態化電位（2）に至るまでの活性態域（ａ）の掃引速度を早くすることで、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化することができる。また、不動態皮膜の生成が始まった当該不動態化電位（2）から折り返し点(R)を経て再不動態化最小電位（8）に至るまでの下降方向の掃引速度を遅くすることで、当該不動態化電位（2）から上記任意値に至るまでの間で腐食対象部位の試験電極８に不動態皮膜を確実に形成するとともに、その後の再不動態化最小電位（8）に至るまでの間で、粒界部の溶解を選択的に促進することができる。このため、結晶粒表面の過剰な金属腐食（金属溶解）を可及的に抑えつつ、当該結晶粒表面に確実にムラなく不動態皮膜を形成でき、もって結晶軸方向がランダムで腐食の進行度合いが異なっている結晶粒相互間にあっても、その表面高さに段差が生じることを可及的に小さく抑えることができる。 Therefore, as described above, in the intergranular corrosion method of the metal surface of the present embodiment, the passive film is applied from the natural potential (1) when sweeping the potential applied to the test electrode 8 which is the corrosion target portion of the metal surface. By accelerating the sweep speed of the active state region (a) until the passivating potential (2) at which the generation of phosphine starts, the corrosion target site can be activated while suppressing the corrosion of the entire crystal grains. . In addition, by slowing down the sweep speed in the downward direction from the passivating potential (2) where the formation of the passivating film has started to the repassivation minimum potential (8) via the turnaround point (R), Between the passivating potential (2) and the above-mentioned arbitrary value, the passivating film is securely formed on the test electrode 8 at the corrosion target site, and the subsequent repassivation minimum potential (8) is reached. Thus, dissolution of the grain boundary part can be selectively promoted. For this reason, while suppressing excessive metal corrosion (metal dissolution) on the crystal grain surface as much as possible, it is possible to form a passive film on the crystal grain surface without any unevenness, so that the crystal axis direction is random and the degree of progress of corrosion. Even if the crystal grains are different from each other, it is possible to suppress the occurrence of a step in the surface height as small as possible.

さらに、当該再不動態化最小電位（8）にて掃引を停止させて保持することなく、結晶粒面に不動態皮膜を形成して、かつ燐等の脆化元素や不純物が偏析する粒界の不動態皮膜は極薄く形成することができる。このため、爾後の自然電位（1）まで戻す間の活性態域（ａ）で、容易にかつ確実に、しかも効率良く、粒界部の不純物が偏析した部位の不動態皮膜を選択的に破壊して当該部位の金属腐食（金属溶解）を促進させて腐食させることができ、もって粒界溝の幅や深さを大きく形成でき、当該粒界溝の計測を容易に、かつ高精度に行わせることができるようになる。 Furthermore, without stopping and maintaining the sweep at the repassivation minimum potential (8), a passivating film is formed on the crystal grain surface, and a brittle element such as phosphorus and impurities segregate. The passive film can be formed very thin. For this reason, in the active state region (a) while returning to the natural potential (1) after dripping, the passive film at the site where the impurities at the grain boundaries segregated easily and reliably and efficiently is selectively destroyed. The metal corrosion (metal dissolution) of the part can be accelerated and corroded, so that the width and depth of the grain boundary groove can be increased, and the measurement of the grain boundary groove can be performed easily and with high accuracy. Will be able to.

なお、上述の実施の形態では、不動態化電位(2)に達してから折り返し点(R)を経て自然電位(1)に戻る迄の掃引速度を遅くするようにしているが、少なくとも折り返し点(R)からの逆掃引時において再不動態化最小電位(8)に至る迄の間の逆掃引速度を遅くすれば良い。また、上述の実施の形態では腐食対象金属としてＣｒ−Ｍｏ−Ｖ鋼を例示したが、本発明はこれに限定されることはなく、金属全般に適用し得る。 In the above-described embodiment, the sweep speed from reaching the passivation potential (2) to returning to the natural potential (1) through the turning point (R) is slowed down, but at least the turning point. What is necessary is just to slow down the reverse sweep speed until it reaches the repassivation minimum potential (8) at the time of the reverse sweep from (R). Moreover, although Cr-Mo-V steel was illustrated as a corrosion object metal in the above-mentioned embodiment, this invention is not limited to this, It can apply to the whole metal.

電気化学的な再活性化法（ＥＰＲ法）で金属表面の粒界を腐食させる場合に用いられている従来からよく知られた、本発明でも使用する装置の概略構成を示す図である。It is a figure which shows schematic structure of the apparatus used also in this invention well-known conventionally used when the grain boundary of a metal surface is corroded by the electrochemical reactivation method (EPR method). 掃引電位と電流密度との関係を示すグラフで、本発明の掃引速度の制御内容を説明する図である。It is a graph which shows the relationship between sweep electric potential and current density, and is a figure explaining the control content of the sweep speed of this invention. 掃引電位と電流密度との関係を示す一般的なアノード分極曲線のグラフである。It is a graph of the general anodic polarization curve which shows the relationship between sweep potential and current density. 金属表面の粒界腐食方法において、その各種の条件を違えて金属表面を腐食させた場合の、再不動態化最小電流密度値Ｉｒと表面粗さＲａ，Ｒｙ，Ｒｚとの関係を対比して示した表である。In the intergranular corrosion method for metal surfaces, the relationship between the repassivation minimum current density value Ir and the surface roughness Ra, Ry, Rz when the metal surface is corroded under different conditions is shown in comparison. It is a table. 金属表面の粒界腐食方法において、その各種の条件を違えて金属表面を腐食させた場合の、粒界溝体積の測定結果を対比して示したものである。In the intergranular corrosion method of the metal surface, the measurement result of the intergranular groove volume when the metal surface is corroded under various conditions is shown in comparison. 金属表面の粒界腐食方法において、その各種の条件を違えて金属表面を腐食させた場合の、粒界溝体積と全腐食電流量との比（粒界溝体積/全腐食電流量）を対比して示したものである。Contrast the ratio of intergranular groove volume to total corrosion current (intergranular groove volume / total corrosion current) when the metal surface is corroded under different conditions in the intergranular corrosion method on the metal surface. It is shown.

符号の説明Explanation of symbols

２電気化学的な再活性化法に用いる装置
４電解液
６電解セル
８試験電極（腐食対象部位）
１０照合電極
１２対極
１４ポテンショスタット
(1) 自然電位
(2) 不動態化電位
(3) 不動態化最小電位
(4) ２次アノードピーク電位
(5) 不動態化中央電位
(6) 過不動態化電位
(8) 再不動態化最小電位
(a) 活性態域
(b) 不動態域（再不動態域）
(c) 過不動態域
(R) 折り返し点 2 Electrochemical reactivation equipment 4 Electrolyte 6 Electrolysis cell 8 Test electrode (corrosion target site)
10 reference electrode 12 counter electrode 14 potentiostat
(1) Self potential
(2) Passivation potential
(3) Passivation minimum potential
(4) Secondary anode peak potential
(5) Passivation median potential
(6) Hyperpassivation potential
(8) Minimum repassivation potential
(a) Active area
(b) Passive zone (repassive zone)
(c) Transpassive zone
(R) Turning point

Claims

金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、
前記自然電位から不動態化電位までの活性態域では、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引する、
ことを特徴とする金属表面の粒界腐食方法。 The corrosion target site on the metal surface is brought into contact with the electrolyte, and an electric potential is applied. The potential is swept in an upward direction from the natural potential to the arbitrary value exceeding the passivation potential by passing through the active state region. After the passive film is formed on the metal surface, the potential is reversely swept in the descending direction to pass through the repassivation region and the reactivation state region and drop to the natural potential, thereby selecting the grain boundary portion on the metal surface. A method of intergranular corrosion of a metal surface that corrodes electrically,
In the active state region from the natural potential to the passivation potential, sweeping at a high rate to activate the corrosion target site while suppressing the corrosion of the entire crystal grains,
A method of intergranular corrosion of a metal surface characterized by the above.

金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、
前記逆掃引時には少なくとも前記再不動態域の再不動態化最小電位までは、粒界部の溶解を促進すべく遅い速度で掃引する、
ことを特徴とする金属表面の粒界腐食方法。 The corrosion target site on the metal surface is brought into contact with the electrolyte, and an electric potential is applied. The potential is swept in an upward direction from the natural potential to an arbitrary value exceeding the passivation potential by passing through the active state region. After the passive film is formed on the metal surface, the potential is reversely swept in the descending direction to pass through the repassivation region and the reactivation state region and drop to the natural potential, thereby selecting the grain boundary portion on the metal surface. A method of intergranular corrosion of a metal surface that corrodes electrically,
At the time of the reverse sweep, at least to the repassivation minimum potential of the repassivation region, the sweep is performed at a slow speed so as to promote dissolution of the grain boundary part.
A method of intergranular corrosion of a metal surface characterized by the above.

金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、
前記不動態域の不動態化電位から自然電位に戻すまでは、粒界部の溶解を促進すべく遅い速度で掃引する、
ことを特徴とする金属表面の粒界腐食方法。 The corrosion target site on the metal surface is brought into contact with the electrolyte, and an electric potential is applied. The potential is swept in an upward direction from the natural potential to an arbitrary value exceeding the passivation potential by passing through the active state region. After the passive film is formed on the metal surface, the potential is reversely swept in the descending direction to pass through the repassivation region and the reactivation state region and drop to the natural potential, thereby selecting the grain boundary portion on the metal surface. A method of intergranular corrosion of a metal surface that corrodes electrically,
Sweeping at a slow rate to promote dissolution of the grain boundary until returning from the passivation potential of the passive region to the natural potential
A method of intergranular corrosion of a metal surface characterized by the above.

金属表面の腐食対象部位を電解液に接しさせて電位を加え、該電位を自然電位から活性態域を通過させて不動態化電位を超えた任意値まで上昇方向に掃引して、腐食対象部位に不動態皮膜を形成した後、該電位を下降方向に逆掃引して再不動態域と再活性態域とを通過させて前記自然電位まで降下させることにより、金属表面の結晶粒界部を選択的に腐食させる金属表面の粒界腐食方法であって、
前記自然電位から不動態化電位までの活性態域では、結晶粒全体の腐食を抑制しながら被腐食対象部位を活性化すべく早い速度で掃引し、
前記不動態化電位から自然電位に戻すまでは、粒界部の溶解を促進すべく遅い速度で掃引する、
ことを特徴とする金属表面の粒界腐食方法。 The corrosion target site on the metal surface is brought into contact with the electrolyte, and an electric potential is applied. The potential is swept in an upward direction from the natural potential to an arbitrary value exceeding the passivation potential by passing through the active state region. After the passive film is formed on the metal surface, the potential is reversely swept in the descending direction to pass through the repassivation region and the reactivation state region and drop to the natural potential, thereby selecting the grain boundary portion on the metal surface. A method of intergranular corrosion of a metal surface that corrodes electrically,
In the active state region from the natural potential to the passivation potential, sweeping at a high speed to activate the corrosion target site while suppressing the corrosion of the entire crystal grains,
Until returning from the passivation potential to the natural potential, sweeping is performed at a slow rate to promote dissolution of the grain boundary part.
A method of intergranular corrosion of a metal surface characterized by the above.

前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点を、不動態化電位での電流密度に等しい電流密度を発生させる電位以下の範囲とした、ことを特徴とする請求項１〜４のいずれかに記載の金属表面の粒界腐食方法。 The turning point from the sweep in the upward direction of the potential to the reverse sweep in the downward direction is set to a range equal to or lower than a potential that generates a current density equal to the current density at the passivating potential. 4. The intergranular corrosion method for a metal surface according to any one of 4 above.

前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点を、過不動態化電位以下とした、ことを特徴とする請求項１〜４のいずれかに記載の金属表面の粒界腐食方法。 The intergranular corrosion of the metal surface according to any one of claims 1 to 4, wherein a turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction is set to be the overpassivation potential or less. Method.

前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点を、不動態化最小電位に至る前の近傍の電位から過不動態化電位以下の範囲内にした、ことを特徴とする請求項１〜４のいずれかに記載の金属表面の粒界腐食方法。 The turning point from the sweep in the upward direction of the potential to the reverse sweep in the downward direction is set within a range below the over-passivation potential from the potential in the vicinity before reaching the minimum passivating potential. Item 5. The intergranular corrosion method for metal surfaces according to any one of Items 1 to 4.

前記電位の上昇方向の掃引から下降方向の逆掃引への折り返し点が、不動態化最小電位以上で過不動態化電位以下の範囲にある、ことを特徴とする請求項１〜４のいずれかに記載の金属表面の粒界腐食方法。 5. The turning point from the sweep in the increasing direction of the potential to the reverse sweep in the descending direction is in the range of not less than the minimum passivating potential and not more than the overpassivation potential. The intergranular corrosion method of the metal surface as described in 2.

前記自然電位から不動態化電位までの活性態域では、その掃引速度を１０mV／sec 〜１００mV／sec とし、前記不動態化電位から前記再不動態域の再不動態化最小電位までは、その掃引速度を０．１mV／sec 〜２mV／secとすることを特徴とする請求項４〜８に記載の金属表面の粒界腐食方法。 In the active state region from the natural potential to the passivation potential, the sweep rate is 10 mV / sec to 100 mV / sec, and from the passivation potential to the minimum repassivation potential in the repassivation region, the sweep rate is set. The method of intergranular corrosion of metal surfaces according to claim 4 to 8, characterized in that 0.1 mV / sec to 2 mV / sec.

前記金属がＣｒ−Ｍｏ−Ｖ鋼等の低合金鋼であることを特徴とする請求項１〜９のいずれかに記載の金属表面の粒界腐食方法。

The method for intergranular corrosion of a metal surface according to any one of claims 1 to 9, wherein the metal is a low alloy steel such as Cr-Mo-V steel.