US2303778A - Soil pipe - Google Patents

Description

Dec. 1, w42. A. WESLEY mgm SOIL PIPE Filed March 4, 1941 Z5/9.1. 5392. MORE NOBLE MORE NOBLE lfjg. 1&9. 4. ESSA/051.5 ZZ

MORE/VOBLE LESS /VO8LE- HTTRNEY v were discontinuities inthe coating metal initially,

' tion metal even faster than would have occurred Patented Dec. l, 1.942

UNITED l STATES PATENT OFFICE- SOIL PIPE 'l Andrew Wesley, Plainfield: N. J., assignorto The International Nickel Company, Inc., New York,

N. Y., a corporation of Delaware Application March 4, 1941, Serial No. 381,627

In Canada February 6, 1941 4 Claims. (Cl. 1381-64) The present kinvention relates to a soil pipe which resists penetration -by corrosion, and, more particularly, to a soil pipe made of composite ferrous layers having a critical relation of potential diierences between -the exposed layer and the under layer in Which the less noble layer is exposed to the corrosive material. It is well known that metal pipes buried in soil l are subject to corrosion which proceeds non-uniformly over' the exposed surfaces.' 'I'he researches of the Underground Corrosion Division of the National Bureau of Standards, for example, have shown that in the corrosion of iron and steel buriedin the soil,'the rate of deepest pitting in most soils is 7 to 10 times greater than the 15 penetration by general corrosion. Non-uniform eating' away of the metal causes far greater ecenonic loss vthan would result from uniform corrosion over the entire exposed area even though the weight of the metal lost in thecJ se20 of uniform attack is greater than in the case of 4localized corrosion. Penetrating pit formation resulting from non-uniform attack materially shortens the useful life of a pipe and enormous outlays must be made 1in-the maintenance of long pipe lines for conveying water, gas, oil, gasoline', etc., where it is the occurrence of perfora tion of the pipe by the deepest -pit rather than the slow general corrosion which determines when a length of pipe must be removed. It has been proposed to construct pipes of alloy steels but experience proved that'any increase in life due to enhanced resistance of such steels to corrosion was too small to be economically justi' able. AThe alloy structural steels, moreover, were susceptible to non-uniform corrosion to much the `same relative degree as plain carbon steels.

It has also been proposed to construct pipes of coated metals having a thin layer of metal more noble than iron or steel which was exposed to soil or other corroding agent. Unfortunately, such composite metals suffered from the disadvantage that their usefulness depended upon the integrity off the corrosion resistant layer because if there or if discontinuities developed in service, so that I the baseor -foundation metal of iron or steel was -exposed to the action of a corroding medium, the attack proceeded at such points vin the foundain the absence of the more noble coating. Insidious in nature, the extent of the attack .in he base metal was often hidden behind the layer of exposed metalv in which a mere pinhole was per. ceptiblewhereas the base structural metal was accompanying drawing in which: j

eaten away over a large area due to the .electrolytic relations of the tWo metals. Inasmuch as the pipes were almost entirely dependent upon the base structural nieta] for strength, detrimen- 5 tal and dangerous weakening of the structure resulted from s h non-uniform corrosion, and in some instance failures occurred and of course leakage developed. The cost of steel coated with more noble metal was-very high compared with l0 plain carbon steel and was practically prohibitive underlying layer. i 'It is an object of the present invention to'. o#

vide a pipe which will be resistant to paifora ion by localized corrosion. v It is another object of the present invention to provide composite ferrous pipes comprising tvvo4 orthree layers of metal having different electrochemical potentials and the less noble metal is exposed to the corrodinagent or agents.

It is l? further object of the invention toprovide pipes made of composite steel plates resistant to penetration bylocalized corrosion in which the srindividual layers of steel forming the composite pipe are of substantial thickness, each layer cou-4 tributing appreciably tothe strength 'of the structure.

, It is also within the. contemplation of the 40 invention to provide pipes in which those parts that `are exposed to corrosion from onlyv one side,

e. g., from the soil, are made of composite metal having two layers of ferrous metal with different. u

electrochemical potentials, the less noble of the two being exposed -to the pitting corrosive.

`The invention likewise contemplates the prof vision of pipes in which those parts that are ex- .posed to pit corrosion from both sides are made of composite metal having three structural layers of ferrous metal, the exposed layers beingv less noble than the central layer;

Othern objects and-advantages of the present .invention will become apparent 'from the -follow- 'ing description taken in condunction with the if r Fig. 3 illustrates a fragmentary longitudinal section of a two layer pipe line adjacent to a welded joint;

Fig. 4 is a cross section of the pipe illustrated in Fig. 3 taken along the line 4 4; and

Fig. 5 depicts a fragmentary longitudinal section of a two layer pipe line adjacent to a coupled joint.

Generally speaking. the invention comprises pipes made of composite structural plates of ferrous metal, preferably steel, the layers of which are suiiiciently thick to contribute appreciably to the strength of the structure and which have diierent electrochemical potentials. The layers are so arranged that the less noble layer is exposed to the pitting corrosive.

Referring now more particularly to Figs. l and 2 of the ldrawing, it will be seen that the fragment of the pipe line illustrated includes two lengths of pipe designated by reference numerals 2 and 4, each of which is made of a composite plate comprising an outer layer 6, an intermediate layer 8 and an inner layer I8. The two lengths 2 and! are joined together by a weld I2. The exposed layers 8 and l0 are of appreciable thickness and strength vand are not to be confused with thin, weak protective layers such as galvanized coatings, electro-deposited coatings, and vthe like. Layers 8 and I0 are respectively exl'posed in use to the corrosive action of the soil and the contents, such as salt water contaminated the production of corrosionpits. In accordance with the principles of the ypresent invention, layers 8 and I8 are made of steel less noble than layer 8. For example, in making a pipe having a wall thickness of about 0.3 inch layer 8 may be made of an alloy steel about 0.16 inch thick containing about 1% ofy copper and about 2% of nickeLAand layers 8 and I0 may be plain carbon to contribute substantially to the strength of the plates making up the walls of the. pipes. The layers 6 and I0 do not consist of a mere thin coating without structural strength, as in the prior art pipes of galvanized or coated steel sheets, but is to be considered as a structural element of the combination. This is commercially prac- `'ticable in pipes embodying the present invention because the lyers a'e composed of relatively inexpensive steels having good mechanical properties. Further and important advantages of pipes embodying the present invention are that there is no danger that the layers B'and I0 will crack or peel off even under severe physical shocksfin handling, joining, etc., that corrosive attack' is confined to the layers 8 and I0, until they have the layer 8 may become exposed to the corroding the pipe may be readily determined by visual inspection since hidden corrosion cannot occur in composite pipes embodying the present invention. It will be observed from the foregoingthat 'the structural layers ofthe pipes embodying the present invention are arranged opposite to that of the prior art clad steel structures in which the more resistant layer had been exposed to the corrosive medium, whether soil or fluid. The prior art arrangement was. of course, the natural and reasonable way to attack the problem in the lightof existing knowledge and experience with the protection of steel againstI atmospheric and many other types of corrosion. Soil pipes, however, are subject to the foregoing peculiar and special corrosive conditions which lead to pit formation, and under these special and unique circumstances a cladding of more vnoble metal detersthe attack on the under layer only a relatively short time, i. e., only until it is perforated by pits, after which the attack on the base metal -proceeds at an accelerated rate due to the unfavorable relationship of electrochemical potentials of the two layers. By reversing the potentials of the exposed and under layers, and by making both' of substantial thickness, the surprising discovery has been made that total life of a pipe lineis increased many times. This new and surprising result is obtained, moreover,

with cheaper materials which are easily produced in structural shapesand plates and readmixtures, which attack metal non-uniformly with i`been substantiallyalleaten away even'. though medium through pits, perforations 'and the like in theulayeri or I8, and that the true condition of ily fabricated into pipes and pipe lines..

While the mechanism of .pit corrosion of duplex pipes having layers of different nobility is perhaps not entirely clear, a probably theoretical explanation isthat when a layer of protective metal, e. g., a metal more noble than a baselayer of carbon steel, was exposed to a corroding me-y dium and the attack proceeded non-uniformly, as it normally does. pits began t'o form in the protective', metal, which eventually reached the lessnoble layer, e. g.. plain carbon steel. As

soon as this layerwas exposed, localized corrosion proceeded rapidly at these points until both layers were perforated. The two metals formed, in eiiect, an electric cell in which the base metal was the anode, the protective metal the cathode and the corrosive medium the electrolyte. The base metal was thus anodically corroded at a relatively rapif ratedepending upon the electrolyte, the difference in the potentials of the two metals, etc.,- as those skilled in the art will readily understand. On the hotherxhand, when the lessnoble layer, e. g., the plain carbon layer,

is exposed to the corroding medium as in pipe A llnesgconstructed in accordance with the present invention andthe attack proceeds non-uniformly. pits begin to form :lust as above and eventually reach the more noble layer, e. g., the above mentioned copper-nickel or nickel steel,.but further penetration is arrested as substantially no corrosion of the under layer of more noble steel can occur until practically all of the outer layer of less noble steel has been corroded away. `In other words, instead of pitfonnation proceeding at an accelerated rate as' soon as the/outer layer isperforated, as in the prior art structures where the more noble layer was iirst exposed to the corrodant,it substantially ceases in 4structures embodying the present invention where the less noble layer is'iirst exposed. Y

The pipe line illustrated in Fig. 1 is adapted forusein situations where the metal is exposed in use to non-uniform corrosion both by the soil and the 'contents ofthe pipe line. Where n ascafzvaA the-contents are not particularly corrosive or` l layers are of sulcient thickness to contribute structurally and substantially to the strength of the pipe..` The desirable ratios of' thicknesseslo between the more noble and less noble layers of the pipe will depend upon relative costs, corrosion rates, the stress imposed, and other practical limitations. Generally speaking, the less noble materialV in duplex pipe will constitute from about ifa to of the total thickness. For example, in making a pipe having a wall thickness of about inch, the outer layer may be made oi' mild steel about 1% inch thick and the inner layer may be made of low alloy steel, such as S. A. E. 2515, of a solution potential more noble than mild steel, also about 1% inch thick. Both layers contribute to the strength of -the pipe and-when the deepest pits penetrate the mllddsteel layer the more noble' alloy steel is exposed but furthery penetration at that point is arrested-or greatly delayed by galvanic pro.

tection of the alloy steel due to itsA contact with the less noble mild steel layer.

The lengths of composite pipe are joined together at their juxtaposed edges o r ends to form the pipe line. As shown in Fig. 3, the length of pipe may be joined by welding and in this instance the weld has been illustrated as comprisingvan outer bead or layer 24 of material substantially corresponding in composition or nobility to the outer layer I8 overlaid on a layer of weld metal 2B having a composition approximately the same as, or at least the order of nobility as the more noble layer 20 oi' the pipe.l

It will be apparent to those skilled in the .art that in welding together three layer pipes of the type illustrated in Fig. 1 a three layer compesite we1a may be utilized in which the myers of the weld correspond I substantially in order of nobility or composition to the layers of the joined pipes. Ordinarily, however, satisfactory results may be obtained in practice using a welding rod corresponding substantially in analysis or order of nobility to the more noble layer of the pipe.

Other types of joints may be used for connecting the lengths of pipe together to form a pipe line. For example, I have illustrated in Fig. 5 a. screwed joint suitable for connecting two lengths of pipe similar to those .illustrated in Fig. 3. These lengths, which are designated 28 and 30 are `proyided with screw threads at their ends and they are connected together by a sleeve coupling 32 in the usual manner. 32 is preferably Amade of composite metal having an under layer 34 and an outer layer 36 correasponding to the composite pipes which are to be joined together. The exposed layer 36.15 made ofthe less noble metal. Where the corrosive conditions are not severe satisfactory results may be had where the more noble metal 34 is exposed, as shown at the left end of the coupling. The exposure of the more noble metal 34 will cause some acceleration of the corrosion of the adjacent outer 'surface of the pipe andV where it is desired to avoid this, the end of the coupling may be spun 'I'he coupling properties are desired, two low alloy steelsdier'- f a in composition to provide the essential differ- Other types of joints. for example, iianged couplings, packed and wiped joints may also be sed and these types of joints involveno deviation om standard 'practice except that in the case of Y ythat the potential diiferencel between the exposed and the under layer be at least large enough to give effective protection. A 2% nickel steel coupled to plain carbon steel gives an adequate potential diference asdo various of the coppernickel steels when coupled to carbon steels. On the other hand, the potential difference should not be so large as to result in excessive corrosion rates `where a portion of the more noble layer is exposed. Large potential differences are not only detrimental from acorrosion standpoint but entail the use of larger percentages of alloying elements and hence increase the cost. In general, low alloy steels appear adequate to provide the necessary difference in nobility. In electrochemical potential tests in which plain carbon steel was coupled with 1% copper-2% nickel st eel, the potential difference was about 0.08 volt in a 3% sodium chloride solution. Correspondingly, the potential diierence when the 5% nickel vsteel was coupled. with plain carbon steel of the` same analysis in the same solution amounted to about 0.11 volt. In general, it is preferable to maintain a potential difference between thelayers of more noble and less noble\steels employed within the range of about 0.07 volt to about 0.15 volt,v

as determined by this test, and for practical purposes the potential difference should never exceed about 0.2 volt.

It is` evident that under the more important soil corrosion conditions the absolute potentials exhibited by diierent .ferrous alloys may cover a considerable range, theimportant requirement being that the ferrous alloys selected for the surface and underlying layer exhibit sumcient difference in their potentials to arrest the progress of pits upon the exposure of the underlying layer,

suflicient nobility to be highly effective as the l more noble layer when coupled with plain carbon steel. Otherv elements such as cobalt, molybdenum and the like which render iron more noble may also be employed in the more noble layer ibut -are rather expensive and, except in instances where their effect upon other properties is of value, they appear less desirable than copper and nickel.

The present invention contemplates the use of two steels orvcast irons of diierent alloy content or structure for the more noble and' less noble layers respectively. Thus; where high mechanical ence in nobility could be employed; for instance, a low copper steel could be employed for the nickel or lwill readily understand. Instead Aofcasting one outer, less noble layer 'or layers and a nickel or copper-nickel steel of higher alloy content than required when carbon steel is employed` forlthe less noble element, could be employed forthe more noble layer, the relative amounts of alloying metalsbeing selected to produce the desired difference of potential between about 0.07 and 0.2 volt, as above described. ,i

The composite pipes of the present invention may be manufactured in any commercial manner. For example, composite pipes may be manufactured lby assembling clean close-fitting seamless tubes of each of the different metals or alloys desired in'the composite pipe' and working them, e. g., by drawing, to produce at least good 15 mechanical and electrical contact. -Preferably tubes of larger diameter than the nal desired size are assembled and hot rolled and/or hot drawn to the desired dimensions. This treatment involving the .simultaneous application of hea-t and pressure results in thorough bonding of the layers of thecomposite pipe. Fitting by shrinking is another method that may be employed in assembling concentric tubes for direct use or for reduction to smaller size. Composite pipes may also be produced by casting directly to finished size, for example, by centrifugal casting. By

properly choosing the compositionof the layers making up the composite pipe, the pipe may be made of cast iron or steel or a combination of' both, e. g., an outer layer of less noble steel lined with a casting of a more noble cast iron or steel,

in accordance with the principles of thepresent invention which have been set forth herein.

Composite pipes embodying the present invention may'also be made, if desired, from composite skelp by welding the longitudinal seams in the manner hereinabove disclosed for the transverse seams I2 and 22:, for example. The composite plates from which the skelp is produced may be made by positioning a slab of low carbon steel having a cleaned surface at one side of a mold with the cleaned surface exposed to the casting cavity remaining at the other side of the mold into which the nickel, nickel-copper, or other alloy steel that is to form the more noble layer is then cast to form a composite ingot. A satisfactory bond between the low carbon steel and the alloy steel is formed in this manner and the composite ingot may be rolled into composite plate by the use of conventional equipment, as those skilled in the art will readily understand. It will be understood by those skilled in the art that the preformed slab that is positioned in the mold could be the alloy steel against which the low carbon steel is cast. Similarly, if a three layer plate is desired, a slab of the 'alloy steel which has been cleaned on both sides may be positioned at the center of a mold with casting cavities at either side into which low carbon steel 6 may be poured to form a three layer composite ingot; or two slabs of the low carbon steel could be placed at the sides of the mold leaving a cast.-

ing cavity between them into which the alloy steel is cast to form a three layer composite ingot. The three layer compositeI ingot may be rolled into three layer composite plate in the conventional manner, as those skilled in the art of the metals against a preformed slab of the.70 other' in the aforesaid manner, the composite platesimay be fabricated-by welding plates of the desird composition together. A well known method that maybe used comprises placing the plates to be welded in superimposed relation with then passing the heated plates between rolls to eiect intimate contact between the juxtaposed faces of the plates whereby welding takes place over the entire contacting areas of the plates and a composite plate is produced. The fact that steels of essentially similar working characteristics can be employed and chromium need not be presentavoids numerous troubles which have been involved in the production of previously known duplex materials. The foregoing methods of producing the composite pipes.or plates to be used in making pipesembodying the present invention are merely illustrative and 4 are not to beconstrued as the only available methods. Any method whereby composite pipes of the foregoing .kind may be produced may be employed to manufacture the composite pipes to be used in the construction of pipe lines embodying the present invention, as those skilled y of low alloy steels which are easily bonded and welded together and readilyV worked to sha'pe.

The layers are of appreciable thickness so that `there is no danger of peeling or' rupture of a coating and each layer contributes structurally to the strength of the pipe.. Even with linexpensive low alloy steels, highly satisfactory resistance to localized corrosion is obtained.

It shouldbe noted that in this Adescription and y in certain'lof the fclaims, the terms low alloy ferrous metal, lo`w alloy iron, and low alloy steel are used not only to designate thoseferrous metals, including irons and steels, having an alloy content effective topalter the electrochemical potential, but also includegplain cast irons and carbon steels. The present invention is a species of the generic invention disclosed and claimed in appli,- cants copending application, Serial No. 207,485;

led May 12, 1938, and which matured .on April 8, 1941, as Patent'No. 2,237,321

Although the invention has considerable detail withrespect to pipe lines as a preferred embodiment, other structures, ob-v jects, etc., which are exposed to similar .conditions are to be considered equivalents and to come 'within lthe scope of the invention as described in the specification and defined by the appended claims. 4

I claim:

1. As an article of manufacture, a composite soil pipe resistant to penetration by pit corrosion when in the soil composed of at least two and not more than three structural iron vbase metal layers, the expose@ layer being less noble than the under'layer by a potential difference of about 0.07 to 0.2 volt,saidlayers being in good electrical contact over substantially their entire" contacting surfaces, said exposed layer being of substantially greater thickness than a-mere coating and both layers contributing structurally and appreciably to the strength of the pipe. k 2. As an article of manufacture, a composite soil pipe resistant to penetration by `pit corrosion when in the soil and being constructed of two structural layers of steel, the'under layer containing alloying material in sufficient amount to render fit more noble-than the outer layer by a been desc'nbed'im potential difference of about 0.07 to 0.2 volt, both layers contributing structurally and appreciably to the strength of the pipe.

3. As an article of manufacture, a composite soil pipe resistant to perforation by pit corrosion when in the soil and when containinga corrosive uid, said pipe being made of three structural layers of steel, the intermediate layer containing alloying material in sucient amount to render said intermediate layer more noble than the outside layers by a potential difference of about 0.07 to 0.2 volt and all layers contributing structurally and appreciably to the strength of 'the pipe.

4. As an article of manufacture, a, composite soil pipe resistant to penetration by pit corrosion when in the soil having an outer structural layer of plain carbon steel exposed to the soil and an under structural layer of steel bonded thereto containing up to about 5% nickel to render the under layer more noble than the outer layer by a potential dilerence of about 0.07 to 0.2 volt, each layer being of substantial thickness and contributing structurally andappreciably to the strength of the pipe.

- ANDREW WESLEY.V

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