CN103267723A - Detection method for metal pipes and container pitting based on field signature method - Google Patents

Abstract

The invention relates to a high-precision detection for metal pipes or container pitting (pitting corrosion), especially to an improved and relatively-accurate detection method for oil and gas pipelines in petrochemical industry or containers with small pits based on a field signature method. A technical scheme provided by the invention uses a primary and secondary voltage method, and concretely comprises: using measuring electrodes corresponding with areas where any corrosion pits are located as main electrodes and corresponding voltage as a main voltage; using n pairs of electrodes adjacent to the main electrode as auxiliary electrodes and corresponding voltage as an auxiliary voltage; building a corrosion database through calculating a constant current field; and matching the measured, treated and corrected change values of the primary and secondary voltages with the database, thereby obtaining area and depth of small corrosion pits in regions corresponding with the main electrodes. By using the method to detect the metal pipelines or the container pitting, a detection precision can be greatly improved.

Description

Metallic conduit, container pitting corrosion detection method based on the field fingerprint technique

Technical field

The present invention relates to the high-precision detecting method of metallic conduit or container inner wall pitting corrosion, specifically, relate to a kind of based on the field fingerprint technique to petrochemical industry oil and gas pipes or the little etch pit of container high-precision test method more, solve the unsolvable problem of the medium and small etch pit of traditional F SM method innovatively, belonged to field of measuring technique.

Background technology

Petrochemical industry generally adopts the corrosion condition of resistance probe method and the online detection pipeline of polarization sonde method at present, but these methods can only be carried out uniform corrosion and be detected, the great pitting corrosion of harmfulness is not had detectability, and the pitting corrosion detection method of high precision and high reliability and technology are one of technology of pressing for of petroleum chemical industry safe operation.The field fingerprint technique (Field Signature Method, FSM) owing to good reliability, high-low temperature resistant, the life-span is long, and the corrosion that has been widely used in oil and gas pipes, important container and structure abroad detects.

For ease of understanding technical scheme of the present invention, first brief introduction FSM principle, development course and existing corrosion parameter are measured, computing method:

FSM is a kind of glitch-free corrosive pipeline detection technique, and its measuring electrode and all support equipments all are installed in detected object (as storage tank, pipeline etc.) outside, and is contactless with the fluid in the detected zone.Because after the corrosive pipeline, this corrosion region (being covered by pair of electrodes) attenuation causes resistance to increase, under the effect of constant current source I, the change in voltage between measurement and analytical measurement electrode obtains the situation of pipeline corrosion.In order to eliminate the influence that temperature and electric current change, must increase a pair of contrast electrode, place one usually and be close to outside and and the homogeneity sheet metal of piping insulation of pipeline.Figure 1 shows that pipeline external pelivimetry electrode and reference electrode synoptic diagram, after pipeline installs, superscribe a layer insulating (as polyester vinyl) again and imbed underground or the seabed with pipeline, need not to excavate burial layer during measurement.

FSM is proposed in a patent first by Norway scholar H.Hannestad1983.It is high especially that this method and technology have reliability, the good and king-sized advantage of temperature accommodation of security.The essence of FSM is the potential method of matrixing, is about to one group of potential electrode matrix and (normally welding) is installed in detected metal object outside, for detection of the corrosion condition of object inside.Its ultimate principle is test metallic object changes in resistance.Because resistance between electrode is minimum is generally tens u Ω, even apply tens of peace electric currents, voltage across poles also has only tens uV, and the change in voltage that causes during corrosion 0.1mm has only the 0.1uV order of magnitude.Be subjected to the restriction of components and parts and measurement instrument industry level at that time, this technology is not applied.

1989, Norway CorrOcean company developed practical FSM product on the basis of buying above patent.A lot of scholars make a large amount of research and contribution to correlation technique.The most representative have:

1991, people such as Norway R.Strommen have proposed the model of improved FSM, by increasing by a reference plate, reduced the influence that temperature and exciting current change, a fingerprint coefficient FC concept and algorithm have been proposed, make pair of electrodes the zonal corrosion amount accuracy of detection that all cover be improved, obtained using more widely thereby FSM is detected in the corrosion of submarine pipeline and seabed important component.Be subjected to the restriction of computing technique at that time, the limitation of FSM on principle also is not realized, and the calculating of pitting corrosion for example when all FC change, adopts 3-5 safety coefficient doubly to provide the pitting corrosion amount still based on experimental formula, makes the pitting corrosion amount not have the precision determinacy.

2008, people such as Britain scholar D.M Farrell and A.Daaland recognized that uniform current will be disturbed, and cause the precise decreasing of system when electric current is regional through pitting corrosion on the basis of simulation calculation.Because the randomness of pitting corrosion size, the degree of depth and position makes the distribution of current field not have determinacy, when accuracy of detection is 10-15%WT(wall thickness 10mm, error 1-1.5mm).

The people such as G.Sposito, Peter Cawley of the science and engineering Imperial College of Britain kingdom in 2010 have recognized because corrosion causes electric current to be redistributed and have caused potential change no longer to be linear problem, and the potential probes layout has been optimized.Under the condition of the existing priori in pitting corrosion position, proposed with the corresponding corrosion area of pair of electrodes voltage and be aided with the variation development that potential diagram (Potential Drop Mapping) detects corrosion, the potential probes spacing optimum solution that provides does not have universality.

(4 o'clock on a line for the four-point method of N.Bowler of American I owa State University in 2009,2 exterior points apply electric current, point is surveyed voltage in 2, be the FSM special case) carried out theoretical analysis, the approximate computing method of pair of electrodes voltage and thickness have been provided, four-point probe principle for layout when having proposed the measurement sheet metal.Four-point method can be measured pitting corrosion, but needs pointwise to move detection, is not suitable for being used in the occasion of real-time detection, for example seabed and buried pipeline.

But up to now, there is undecidable decision problem in the FSM method for the detection of little etch pit.

The definition of little etch pit is: the zone that the area of etch pit covers less than counter electrode.Because pitting corrosion must be by not having to having, having shallow to dark, so little etch pit must exist.When little etch pit occurring between a pair of potential electrode, the interpolar bulk resistor can increase, simultaneously, the distribution of electric current also can change, thereby cause voltage across poles to change, but a little etch pit has two factor affecting the variation of voltage---area and the degree of depth in hole, that is:

ΔV _i,j;i,j+1＝f(S,D)

S---area, D---the degree of depth.

Because corresponding potential electrode can only be exported a voltage change, thereby the above-mentioned relation formula is separates more, the area and the degree of depth that are little etch pit have multiple possibility, only can't determine area and the degree of depth of little etch pit with a voltage change, and namely little etch pit can not identification problem (Fig. 3).

Disposal route in the past is to eliminate a variable artificially, area S is considered as a fixed value, the area size definition that is about to etch pit is that a potential electrode is to corresponding border circular areas or rectangular area, the diameter that this is regional or the length of side equal the right anode-cathode distance of potential electrode (Fig. 4), with all potential electrode of voltage instrumentation amount to and reference electrode to (t when the no etch pit ₀(t constantly) and when etch pit is arranged _xVoltage constantly), according to resistance chain model shown in Figure 2, arbitrary local corrosion degree to the potential electrode representative is judged by fingerprint coefficient (FC value):

${\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)=(\frac{\frac{V_{i,j;i,j+1}\left(t_x\right)}{V_{i,j;i,j+1}\left(t_0\right)}}{\frac{V_{\mathrm{ref}}\left(t_x\right)}{V_{\mathrm{ref}}\left(t_0\right)}}-1)\×1000$

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly;

V _Ref(t ₀), V _Ref(t _x)-normal electrode is at t ₀And t _xVoltage constantly;

According to the FC value, can obtain wall thickness WT(Wall Thickness) computing formula:

${\mathrm{WT}}_{i,j;i,j+1}\left(t_0\right)=\frac{{\mathrm{WT}}_{i,j;i,j+1}\left(t_0\right)\×1000}{{\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)+1000}$

In the formula: WT _{I, j; I, j+1}(t _x) be current tested area thickness, WT _{I, j; I, j+1}(t ₀) be tested regional original thickness.

Said method can not reflect the actual state of etch pit truly, exactly, the depth error of the little etch pit that solves according to this formula will reach ± and 15～25%WT(WT is wall thickness, when wall thickness was 10mm, precision was ± 1.5～2.5mm, and the etch pit area that I is surveyed is 1.5WT).

Summary of the invention

The present invention is directed to present FSM method and wait not enoughly to the pitting corrosion accuracy of detection is low, a kind of pitting corrosion of accurately differentiating is provided, the measuring method of particularly little etch pit, thereby development and improve the FSM method.

Technical scheme of the present invention is:

A kind of metallic conduit, receptacle hole shape corrosion detecting method, based on field fingerprint technique principle, outer wall at metallic conduit or container is arranged the potential electrode matrix, described potential electrode matrix has i row electrode at the circumferencial direction of pipeline, at axis direction the j column electrode is arranged, it is right that two adjacent electrodes that arbitrary axis makes progress constitute a potential electrode; A reference plate is set, and the material of this reference plate is identical with the material of tested pipeline or container; Described reference plate is provided with a pair of reference electrode, as the reference standard electrode; The potential electrode of region, definition random corrosion hole is to as central electrode, and corresponding voltage is as principal voltage; The n axial electrode adjacent with this central electrode is to as auxiliary electrode, and corresponding voltage is as auxilliary voltage; Measure as follows again and find the solution area and the degree of depth of etch pit:

(1), the measured zone with the central electrode correspondence is subdivided into a * b residential quarter again; With electrode separation 1/5～1/3 as pitting corrosion diameter initial value, the maximal value of electrode separation is pitting corrosion diameter stop value, step-length is 0.5～1mm, with 1/20～1/15 wall thickness as pitting corrosion degree of depth initial value, 7/10～8/10 wall thickness is pitting corrosion degree of depth stop value, step-length is 0.5～1mm, utilizes steady current field method, calculates 1～ab residential quarter (t when not having etch pit one by one ₀(t constantly) and when etch pit is arranged _xPrincipal voltage V constantly) _{I, j; I, j+1}, auxilliary voltage V _{I-1, j; I-1, j+1}, V _{I+1, j; I+1, j+1}..., V _{I, j+n; I, j+n+1}Deng, set up the matching database of major-minor voltage method;

(2), from reference plate to metallic conduit or container input constant current drive electric current, make this exciting current flow through reference electrode and potential electrode matrix, the strength of current of the intensity of this exciting current when adopting the method calculating of steady current field is identical;

(3), with all potential electrode of voltage instrumentation amount to and reference electrode to (t when the no etch pit ₀(t constantly) and when etch pit is arranged _xVoltage constantly);

(4), be calculated as follows t _xFingerprint coefficient constantly

${\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)=(\frac{\frac{V_{i,j;i,j+1}\left(t_x\right)}{V_{i,j;i,j+1}\left(t_0\right)}}{\frac{V_{\mathrm{ref}}\left(t_x\right)}{V_{\mathrm{ref}}\left(t_0\right)}}-1)\×1000$ (unit: ppt, Partperthousand) (1)

In the formula:

V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly;

V _Ref(t ₀), V _Ref(t _x)-reference electrode is at t ₀And t _xVoltage constantly;

(5), obtain revised principal voltage by following formula:

V _i,j;i,j+1(t _x)＝FC _i,j;i,j+1(t _x)×V _i,j;i,j+1(t ₀) （2）

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-central electrode is to (i, j; I is j+1) at t ₀And t _xVoltage constantly; Auxilliary voltage solution formula is identical with the principal voltage solution formula, and solves auxilliary voltage;

(6), obtain revised principal voltage changing value by following formula:

ΔV _i,j;i,j+1(t _x)＝V _i,j;i,j+1(t _x)-V _i,j;i,j+1(t ₀) （3）

Auxilliary voltage change solution formula is identical with principal voltage changing value solution formula, and solves auxilliary voltage change;

(7), with revised major-minor voltage change and corrosion data storehouse coupling, namely area and the degree of depth of the etch pit hour of the major-minor voltage phase difference in revised major-minor voltage and the database are the value of finding the solution.

As preferably, described measured zone is subdivided into 3 * 3 totally 9 residential quarters, i.e. a=b=3.

As preferably, described voltmeter adopts high-resolution accurate voltage measurement module.

Beneficial effect of the present invention: the present invention is under the situation that does not change original FSM method probe layout, by major-minor voltage method can the little etch pit of rapid solving area and the degree of depth, and increased substantially measuring accuracy, make measurement result reflect the actual state of hole corrosion more truly, exactly, provide reliable basis for people judge accident potential.

Description of drawings

Fig. 1 FSM method principle schematic

Fig. 2 FSM equivalent resistance network model and major-minor voltage synoptic diagram

The little etch pit undecidable decision problem of Fig. 3 synoptic diagram

The contour shape synoptic diagram of the pitting corrosion that Fig. 4 traditional F SM method is thought (this method thinks that the area S of pitting corrosion fixes)

Equivalent resistance network model figure after Fig. 5 FSM segmentation

Embodiment

In order accurately to differentiate area and the degree of depth of little etch pit, the present invention proposes a kind of major-minor voltage method, namely defines the corresponding electrode pair in random corrosion position, place, hole as central electrode, and corresponding voltage is as principal voltage V _{I, j; I, j+1}The axial electrode of the n that this central electrode is adjacent is as auxiliary electrode, and corresponding voltage is namely assisted voltage V as auxilliary voltage _{I-1, j; I-1, j+1}, V _{I+1, j; I+1, j+1}..., V _{I, j+n; I, j+n+1}Deng; Utilize steady current field method to set up the corrosion data storehouse simultaneously, related request according to steady current field method applies the constant current drive electric current to tested pipeline or container, measure and handle and obtain revised major-minor voltage change, by mating major-minor voltage change, thus high precision and determinacy is arranged obtain area and the degree of depth that the pitting in this central electrode corresponding region corrodes.Its principle is as follows:

Equivalent resistance chain model among Fig. 2 is required to carry out the ab uniform subdivision according to measuring, namely be subdivided into ai * bj sub-resistance (Fig. 5), then a resistance can be subdivided into ab sub-resistance, calculates the FC of segmentation resistor network than value array, is drawn by following formula:

$\frac{{\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)}{{\mathrm{FC}}_{i,j;i,j+1}\left(t_0\right)}=\frac{\frac{I_{\left(n\right)(n+1)}R}{\mathrm{IR}}-1}{\frac{I_{(m-1)\left(m\right)}(R+R^{\text{'}})}{\mathrm{IR}}-1}$

$=\frac{I_{\left(n\right)(n+1)}R-\mathrm{IR}}{I_{(m-1)\left(m\right)}(R+R\text{'})-\mathrm{IR}}$

Wherein I is not for the electric current that does not flow through on the preceding every capable resistance chain of corrosion, and R is original resistance value, R' resistance value for a change, I _{(m-1) m}For flowing through the electric current of the resistance that resistance changes, I _{N (n+1)}For flowing through the electric current of the resistance that other resistance do not change, FC _{I, j; Ij ,+1}(t _x) the FC value of the resistance that changes for resistance, FC _{I, j; I, j+1}(t ₀) the FC value of the resistance that do not change for resistance.

Obtain the expression formula of any time voltage across poles according to FC than value array, obtained by following formula:

$V_{i,j;i,j+1}\left(t_x\right)=\underset{x=1}{\overset{\mathrm{ab}}{\mathrm{\Σ}}}\frac{{\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)}{{\mathrm{FC}}_{i,j;i,j+1}\left(t_0\right)}\×\mathrm{\Δ}{V}_{\mathrm{FC}0}+V_{i,j;i,j+1}\left(t_0\right)$

Δ V _FC0The voltage at the resistance two ends that change for resistance, V _{I, j; I, j+1}(t _x) be t _xPrincipal voltage constantly, V _{I, j; I, j+1}(t ₀) be t ₀Principal voltage constantly, ab represents after the former resistor network segmentation, the sub-resistance number of a resistance correspondence.The computing formula of other n auxilliary voltage is also similar.Calculate the principal voltage V after a sub-resistance changes separately _{I, j; I, j+1}(1) and n auxilliary voltage V _{I-1, j; I-1, j+1}(1), V _{I+1, j; I+1, j+1}(1) ..., V _{I, j+n; I, j+n+1}(1), calculates principal voltage V after several sub-resistance change simultaneously again _{I, j; I, j+1}(n) and n auxilliary voltage V _{I-1, j; I-1, j+1}(n), V _{I+1, j; I+1, j+1}(n) ..., V _{I, j+n; I, j+n+1}(n).Show according to result of calculation: when the principal voltage of two kinds of situations equated, auxilliary voltage was unequal, that is:

V _i,j;i,j+1(1)＝V _i,j;i,j+1(n)

V _{i-1,j;i-1,j+1}(1)≠V _{i-1,j;i-1,j+1}(n)

V _{i+1,j;i+1,j+1}(1)≠V _{i+1,j;i+1,j+1}(n)

.

.

.

V _{i,j+n;i,j+n+1}(1)≠V _{i,j+n;i,j+n+1}(n)

Following formula understands in theory that namely different holes may be consistent to the influence of principal voltage, but is inconsistent to the influence of auxilliary voltage.According to above-mentioned theory, can utilize comprehensive area S and the depth D of judging little etch pit of major-minor voltage.

Concrete grammar is:

According to the material of tested pipeline, cast, diameter, thickness, design potential electrode matrix.Circumferencial direction at pipeline is capable with i, arranges with the j row along axis direction; Add a reference plate, and arrange a pair of reference electrode at it along the direction of current mid line region.The horizontal and vertical distance of all electrodes is 3～5 times of wall thickness (WT);

Apply the constant current drive electric current from reference plate, gather and preserve potential electrode group voltage matrix and reference plate voltage then at no etch pit (t ₀(t constantly) and when etch pit is arranged _xVoltage constantly), obtain fingerprint coefficient FC value:

${\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)=(\frac{\frac{V_{i,j;i,j+1}\left(t_x\right)}{V_{i,j;i,j+1}\left(t_0\right)}}{\frac{V_{\mathrm{ref}}\left(t_x\right)}{V_{\mathrm{ref}}\left(t_0\right)}}-1)\×1000$

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly;

V _Ref(t ₀), V _Ref(t _x)-normal electrode is at t ₀And t _xVoltage constantly;

According to the FC value, be calculated as follows and obtain revising the principal voltage changing value:

V _i,j;i,j+1(t _x)＝(FC _i,j;i,j+1/1000+1)×V _ref(t _x)×V _i,j;i,j+1(t ₀)/V _ref(t ₀)

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly; Revise auxilliary voltage

The derivation of equation is similar to correction principal voltage solution formula, and solves the auxilliary voltage of correction;

Calculate the principal voltage changing value according to following formula:

ΔV _i,j;i,j+1(t _x)＝V _i,j;i,j+1(t _x)-V _i,j;i,j+1(t ₀)

Auxilliary voltage change solution formula is identical with principal voltage changing value solution formula, and solves auxilliary voltage change;

Utilize steady current field method to calculate, also verify by experiment and set up the corrosion data storehouse that the data of calculating comprise:

The circumferencial direction of For i=1to i(pipeline has i row electrode)

For j=1to j(has the j capable at the axis direction of pipeline)

For q=1to ab (there is a * b ab residential quarter altogether the principal voltage corresponding region)

For Φ=1/5～1/3d to d, step0.5～1mm (Φ-pitting corrosion diameter, d-electrode separation.)

For D=1/20～1/15 WT to7/10-8/10WT, step 0.5～1mm(D-pitting corrosion degree of depth.

The minimum pitting corrosion degree of depth that the petrochemical industry pipeline need detect is 1/20～1/15WT.）

Goto " calculates V _{I, j; 1, j+1}(t _x) module "

Next Φ

Next D

Next q

Next j

Next i

Actual measurement and revised major-minor voltage and database are mated, can draw area and the degree of depth of etch pit.Coupling adopts the pattern match principle, and establishing the major-minor voltage change of database is Δ V _{I, j; I, j+1}(m), auxilliary voltage change is Δ V _{I-1, j; I-1, j+1}(m), Δ V _{I+1, j; I+1, j+1}(m) ..., Δ V _{I, j+n; I, j+n+1}(m), measuring gained principal voltage changing value is Δ V _{I, j; I, j+1}With auxilliary voltage change be Δ V _{I-1, j; I-1, j+1}, Δ V _{I+1, j; I+1, j+1}..., Δ V _{I, j+n; I, j+n+1}, then have:

Δ ₁＝ΔV _i,j;i,j+1(m)-ΔV _i,j;i,j+1

Δ ₂=ΔV _{i-1,j;i-1,j+1}(m)-ΔV _{i-1,j;i-1,j+1}

Δ ₃=ΔV _{i+1,j;i+1,j+1}(m)-ΔV _{i+1,j;i+1,j+1}

.

.

.

Δ _n=ΔV _{i,j+n;i,j+n+1}(m)-ΔV _{i,j+n;i,j+n+1}

${\mathrm{\&mu;}}_m=\sqrt{{{\mathrm{\&Delta;}}_1}^2+{{\mathrm{\&Delta;}}_2}^2+{{\mathrm{\&Delta;}}_3}^2+...+{{\mathrm{\&Delta;}}_{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00003057200400021.png"\; state="\{\{state\}\}">n</figure-callout>}}}^2}$

At a series of μ _mIn, minimum μ _mCorresponding etch pit namely is solving result.The area and the degree of depth that are the etch pit that differs hour of major-minor voltage change in revised major-minor voltage change and the database are the value of finding the solution.

Specify the present invention with an example below.

Press array of measure electrodes on the certain way cloth at tested metallic conduit outer wall, the spacing of electrode is generally got 3～4 times of wall thickness.Wall thickness WT=10mm in this example, the spacing d=30mm of potential electrode.

Tested metallic conduit is applied dc constant current power supply, and the electric current that this example applies is 2A, through the sampling of accurate voltage repeatedly, obtains potential electrode matrix and reference electrode after handling through data to (t when the no etch pit again ₀Magnitude of voltage constantly).

Carry out the calculating of steady current field according to potential electrode and tested pipeline parameter, set up corrosion data storehouse (partly):

Three points at inner-walls of duct grind 3 holes with bistrique, and its size is respectively Φ ₁=11mm, D ₁=3.2mm, Φ ₂2=16mm, D ₂=1.8mm, Φ ₃=30mm, D ₃=0.8mm (Φ is diameter, and D is the degree of depth).

Carry out repeatedly accurate voltage sampling again, data are handled and are drawn potential electrode matrix and reference electrode to (t when etch pit is arranged then _xVoltage constantly).

Right voltmeter is calculated the FC value to utilize potential electrode group voltage matrix and reference electrode:

${\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)=(\frac{\frac{V_{i,j;i,j+1}\left(t_x\right)}{V_{i,j;i,j+1}\left(t_0\right)}}{\frac{V_{\mathrm{ref}}\left(t_x\right)}{V_{\mathrm{ref}}\left(t_0\right)}}-1)\&times;1000$

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly;

V _Ref(t ₀), V _Ref(t _x)-normal electrode is at t ₀And t _xVoltage constantly;

According to the reduction of FC value, be calculated as follows and obtain revising voltage:

V _i,j;i,j+1(t _x)＝(FC _i,j;i,j+1/1000+1)×V _ref(t _x)×V _i,j;i,j+1(t ₀)/V _ref(t ₀)

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly; Auxilliary voltage solution formula is similar to the principal voltage solution formula, and solves auxilliary voltage;

Obtain the principal voltage changing value again:

ΔV _i,j;i,j+1(t _x)＝V _i,j;i,j+1(t _x)-V _i,j;i,j+1(t ₀)

Auxilliary voltage change solution formula is similar to principal voltage changing value solution formula, and solves auxilliary voltage change;

Thereby obtain the major-minor voltage change in three holes:

Actual measurement and revised major-minor voltage and database are mated, can draw area and the degree of depth of etch pit.Coupling adopts the pattern match principle, and establishing the major-minor voltage change of database is Δ V _{I, j; I, j+1}(m), auxilliary voltage change is Δ V _{I-1, j; I-1, j+1}(m), Δ V _{I+1, j; I+1, j+1}(m) ..., Δ V _{I, j+n; I, j+n+1}(m), measuring gained principal voltage changing value is Δ V _{I, j; I, j+1}With auxilliary voltage change be Δ V _{I-1, j; I-1, j+1}, Δ V _{I+1, j; I+1, j+1}..., Δ V _{I, j+n; I, j+n+1}, then have:

Δ ₁＝ΔV _i,j;i,j+1(m)-ΔV _i,j;i,j+1

Δ ₂=ΔV _{i-1,j;i-1,j+1}(m)-ΔV _{i-1,j;i-1,j+1}

Δ ₃=ΔV _{i+1,j;i+1,j+1}(m)-ΔV _{i+1,j;i+1,j+1}

.

.

.

Δ _n=ΔV _{i,j+n;i,j+n+1}(m)-ΔV _{i,j+n;i,j+n+1}

${\mathrm{\&mu;}}_m=\sqrt{{{\mathrm{\&Delta;}}_1}^2+{{\mathrm{\&Delta;}}_2}^2+{{\mathrm{\&Delta;}}_3}^2+...+{{\mathrm{\&Delta;}}_{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00003057200400021.png"\; state="\{\{state\}\}">n</figure-callout>}}}^2}$

At a series of μ _mIn, minimum μ _mCorresponding etch pit namely is solving result.The area and the degree of depth that are the etch pit that differs hour of major-minor voltage change in revised major-minor voltage change and the database are the value of finding the solution.

Through overmatching, the matching result in three holes is as follows:

The depth capacity error σ that obtains cheating by coupling _Max=± 1%WT(WT is wall thickness), far below the error of conventional fingerprint technique ± 15～25%WT.

Claims (3)

1. a metallic conduit, receptacle hole shape corrosion detecting method, based on field fingerprint technique principle, outer wall at metallic conduit or container is arranged the potential electrode matrix, described potential electrode matrix has i row electrode at the circumferencial direction of pipeline, at axis direction the j column electrode is arranged, it is right that two adjacent electrodes that arbitrary axis makes progress constitute a potential electrode; A reference plate is set, and the material of this reference plate is identical with the material of tested pipeline or container; Described reference plate is provided with a pair of reference electrode, as the reference standard electrode; The potential electrode of region, definition random corrosion hole is to as central electrode, and corresponding voltage is as principal voltage; The n axial electrode adjacent with this central electrode is to as auxiliary electrode, and corresponding voltage is as auxilliary voltage; Measure as follows again and find the solution area and the degree of depth of etch pit:

(1), the measured zone with the central electrode correspondence is subdivided into a * b residential quarter again; With electrode separation 1/5～1/3 as pitting corrosion diameter initial value, the maximal value of electrode separation is pitting corrosion diameter stop value, step-length is 0.5～1mm, with 1/20～1/15 wall thickness as pitting corrosion degree of depth initial value, 7/10～8/10 wall thickness is pitting corrosion degree of depth stop value, step-length is 0.5～1mm, utilizes steady current field method, calculates 1～ab residential quarter (t when not having etch pit one by one ₀(t constantly) and when etch pit is arranged _xPrincipal voltage V constantly) _{I, j; I, j+1}, auxilliary voltage V _{I-1, j; I-1, j+1}, V _{I+1, j; I+1, j+1}..., V _{I, j+n; I, j+n+1}Deng, set up the matching database of major-minor voltage method;

(2), from reference plate to metallic conduit or container input constant current drive electric current, make this exciting current flow through reference electrode and potential electrode matrix, the strength of current of the intensity of this exciting current when adopting the method calculating of steady current field is identical;

(3), with all potential electrode of voltage instrumentation amount to and reference electrode to (t when the no etch pit ₀(t constantly) and when etch pit is arranged _xVoltage constantly);

(4), be calculated as follows t _xFingerprint coefficient constantly

${\mathrm{FC}}_{i,j;i,j+1}\left(t_x\right)=(\frac{\frac{V_{i,j;i,j+1}\left(t_x\right)}{V_{i,j;i,j+1}\left(t_0\right)}}{\frac{V_{\mathrm{ref}}\left(t_x\right)}{V_{\mathrm{ref}}\left(t_0\right)}}-1)\&times;1000$ (unit: ppt, Part per thousand) (1)

In the formula:

V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-electrode pair (i, j; I is j+1) at t ₀And t _xVoltage constantly;

V _Ref(t ₀), V _Ref(t _x)-reference electrode is at t ₀And t _xVoltage constantly;

(5), obtain revised principal voltage by following formula:

V _i,j;i,j+1(t _x)＝FC _i,j;i,j+1(t _x)×V _i,j;i,j+1(t ₀) （2）

In the formula: V _{I, j; I, j+1}(t ₀), V _{I, j; I, j+1}(t _x)-central electrode is to (i, j; I is j+1) at t ₀And t _xVoltage constantly; Auxilliary voltage solution formula is identical with the principal voltage solution formula, and solves auxilliary voltage;

(6), obtain revised principal voltage changing value by following formula:

ΔV _i,j;i,j+1(t _x)＝V _i,j;i,j+1(t _x)-V _i,j;i,j+1(t ₀) （3）

Auxilliary voltage change solution formula is identical with principal voltage changing value solution formula, and solves auxilliary voltage change;

(7), with revised major-minor voltage change and corrosion data storehouse coupling, namely area and the degree of depth of the etch pit hour of the major-minor voltage phase difference in revised major-minor voltage and the database are the value of finding the solution.

2. metallic conduit according to claim 1, receptacle hole shape corrosion detecting method is characterized in that, described measured zone is subdivided into 3 * 3 totally 9 residential quarters, i.e. a=b=3.

3. metallic conduit according to claim 1, receptacle hole shape corrosion detecting method is characterized in that, described voltmeter adopts high-resolution accurate voltage measurement module.

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