AWS PRGC-1999 The Practical Reference Guide for Corrosion of Welds - Causes and Cures《焊点腐蚀的实践参考指南-成因和恢复》.pdf

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1、The PracticalReference Guide forCauses and CuresCorrosionof Welds550 N.W. LeJeune Road, Miami, Florida 33126THE PRACTICALREFERENCE GUIDEforCORROSION OF WELDSCAUSES AND CURESTed V. WeberPrincipal ConsultantWeber online: http:/ 1999 by the American Welding Society. All rights reserved.Printed in the U

2、nited States of America.iiiTABLE OF CONTENTSPage No.Introduction1Corrosion 1General Corrosion 2Pitting Corrosion .6Intergranular Corrosion7Selective Leaching .9Stress Corrosion Cracking9Erosion/Corrosion 12Crevice Corrosion14Galvanic Corrosion16Miscellaneous Corrosion Mechanisms.18Alloying for Corro

3、sion Resistance 21Case Histories.22Corrosion of WeldsCauses and CuresAWS Practical Reference Guide 1IntroductionCorrosion, resulting in the severe degradation ofmaterials, is one of the most expensive engineeringproblems in our industrial society; estimates havebeen made that the annual cost of corr

4、osion in theU.S. exceeds 100 billion dollars. As you might imag-ine with the financial stake so high, manufacturersand end users expend significant amounts of en-gineering time and money avoiding, protectingagainst, or repairing damages from corrosion. Avery large majority of industry must, or shoul

5、d,consider the environmental effects on its productsor equipment and guard against premature failuredue to corrosion.Most of us are aware of a very common corrosionproblem, corrosion of our automobiles. If you havelived near a coastal water area, or in the snow beltareas that keep the roads clear of

6、 ice and snow byspreading salt on the highways each winter, you areprobably familiar with the rusted-out car bodiesthat occur quite quickly when exposed to the moistsalt-air environment. Our automakers spend con-siderable effort to protect their products from frameand body corrosion by these severe

7、environments,but quite often, Mother Nature wins the battle.Other common examples of items needing corrosionprotection include bridges, electrical connections inappliances and electronic devices, chemical process-ing plants, water pipes, hot water heaters, structuralsteel, welds; the list is endless

8、. Suffice it to say thatcorrosion is a pervasive problem and industry mustfirst understand corrosion before they can take stepsto solve the unique problems it presents.Welded structures are often subjected to corrodingenvironments; in some cases, the weld and basemetal corrode uniformly at the same

9、rate. In othercases, the results are accelerated corrosion of theweld compared to the base metal, or the base metalmay corrode at a much faster rate leaving the weldmetal relatively intact. Welding, and its associatedheat input, can also contribute to other corrosionproblems; these will be discussed

10、 in greater detaillater.A logical starting point for dealing with corrosionof welds is to define corrosion and then list thevarious types of corrosion that can occur, with ex-amples. Definitions used are those developed byDr. Mars Fontana and/or NACE. There are manydifferent forms of corrosion recog

11、nized that includethe various corrosion mechanisms, and the mostcommon are defined and discussed below. The nextlogical step after defining the various forms of cor-rosion is to then develop methods for avoidance ofeach and these are also listed following each defini-tion. Several case histories are

12、 also noted to demon-strate actual, practical solutions.CorrosionCorrosion has been defined as “The destruction of ametal by chemical or electrochemical reactions with itsenvironment.” In todays world of ever-increasingmaterials available to the designer, this definitionmust be expanded to cover oth

13、er materials in addi-tion to metals since environmental failures of non-metallics also occur. And since many nonmetallicsare joined (welded), the degradation of nonmetal-lics must be dealt with when selecting them for aparticular application. Since our primary interest isin weld corrosion of metals,

14、 the discussion of weldcorrosion will be limited to consideration of metalsonly.Figure 1. Carbon steel pipe butt joint weld showing severe internal corrosion of weld root and heat affected zone.2 AWS Practical Reference GuideCorrosion of WeldsCauses and CuresWithin metallic systems exposed to aqueou

15、s corrosiveenvironments, anodes (the positive electrode) andcathodes (the negative electrode) of different poten-tials, or voltages, are formed. These areas of differingpotentials can be formed several ways: by dissimilarFigure 2. Section of stainless steel piping showing severe corrosion of weld ro

16、ot. Arrow points to an area of complete perforation.Figure 3. Pipe section showing localized corrosion of longitudinal seam weld.alloys used in combination, by weld metal versusbase metal, by mill scale versus clean metal surfaces,by segregated constituents within a metal structure,or by concentrati

17、on cells within the corrosive environ-ment itself. There are several general statements thatcan be made regarding metal corrosion: Corrosion of metals always occurs at the anode(oxidation), with the cathode (reduction) beingprotected. There is electron flow between the anode andcathode during corros

18、ion. Nascent, or atomic, hydrogen (H+) is alwaysformed at the cathode during corrosion.With these general truths in mind, we will nowmove to the various forms of corrosion.General CorrosionThe most common form of corrosion is referred toas General Corrosion; an alternate term is UniformAttack. One f

19、ormal definition is “A chemical or elec-trochemical reaction which proceeds uniformly over theentire exposed surface, or over a large percentage of theexposed surface.” During this type of corrosion, thematerial corrodes quite evenly over its surface, orthe majority of its surface, that is exposed t

20、o the cor-roding environment.Photo courtesy of Don Johnson, Mt. Juliet, TN.Figure 4. Severe general corrosion of a very old cast steel cannon barrel recovered after many years of being submerged in seawater.Note heavy scale of iron oxides that have completely filled the cannon bore.Corrosion of Weld

21、sCauses and CuresAWS Practical Reference Guide 3A piece of 10 gauge, uncoated, plain carbon steelleft outside and exposed to the rain and sun willusually corrode in a general manner with time. Theentire surface begins to “rust” and turn a dark red-dish brown color (see Figure 5). If sufficient timep

22、asses, the original metal thickness is reduced in auniform or general manner, and eventually the en-tire piece will transform back to its natural state ofiron oxides. The steel combines with oxygen andhydrogen during the corrosion process, which isaided by moisture. The iron oxides and iron hy-droxi

23、des form a scale on the surface and while thescale would seem to offer protection from furtherFigure 5. A carbon steel piping exterior showing moderate general corrosion of the pipe exterior and butt joint weld.Figure 6. Bulge in boiler tube caused by loss of wall thickness by general corrosion and

24、internal pressure on the thinned pipe wall.corrosion, it does not in this example. In fact, theporous and relatively lightly adhering scale of ironoxide and iron hydroxide may even promote fastercorrosion by absorbing moisture and trapping it inclose contact with the steel. (Protective corrosionscal

25、es will be discussed in greater detail when wereview corrosion of the austenitic stainless steels.)Generally, when carbon steels corrode uniformly andcorrosion scales form, the thickness of the scale can beabout 7 to 10 times as thick as the original metal thick-ness consumed to form the scale. Thus

26、, a 1/4 inchsteel plate completely corroded can form a scale of1.5 inches to 2 inches or more. Aluminum alloys alsocan form thick scales in some exposures, and thisscale formation has been termed “blooming.” Thesealuminum scales are quite loose, almost fluffy innature, are very unprotective, and can

27、 be easily re-moved with a little light mechanical scraping.Many other examples of general corrosion can belisted. A few are: copper in a weak nitric acid solu-tion will dissolve; steel immersed in sea watereventually becomes entirely consumed; aluminumin a caustic solution will dissolve; water pipe

28、s inhouses become quite thin and fail; bolted connec-tions suffer corrosion and fail.Figure 7. Metallurgical mount of carbon steel plate cross-section showing depth of oxidation scaling at arrows due to exposure to hot gases.4 AWS Practical Reference GuideCorrosion of WeldsCauses and CuresFigure 9.

29、Shank of stud and nut showing severe general corrosion of stud.We usually describe the rate of general corrosion interms of the thickness lost per unit time. A commonterm in the U.S. is mils per year (mpy) thicknessloss. A mil is 0.001 inch, and an acceptable corrosionrate for steel used in common a

30、pplications such asstorage tanks or petrochemical equipment is usually10 mpy or less, depending on the particular applica-tion. Corrosion data are also given in inches peryear (ipy), but this approach becomes a bit impracti-cal when dealing with normally acceptable corro-sion rates. To demonstrate t

31、his, a 7 mpy rate canalso be written as 0.007 ipy, requiring the use ofseveral zeroes and a decimal point in the ipy format.Corrosion rates can be determined by exposing aspecimen to the corroding environment and measur-ing the actual metal thickness loss with time. Corro-Figure 8. Curves showing we

32、ight gain in 1,000 hour hot air exposure tests for three materials. The carbon steel begins to oxidize (gain weight) at about 900F and begins to oxidize very rapidly. The 12 Cr alloy and the 304 stainless steel have much better oxidation resistance and are useful at much higher temperatures.OXIDATIO

33、N IN 1,000 HOURS050100150200250300350900 1000 1100 1200 1300 1400 1500 1600 1700TEMPERATURE (F)mg per square cm102012 Cr304Corrosion of WeldsCauses and CuresAWS Practical Reference Guide 5sion rates are more commonly determined in lab orfield corrosion testing by weighing the metal speci-men before

34、and after exposure. Then, with a formulausing the measured weight loss, exposure time, ex-posed specimen area, and a material density factor,the corrosion rate in mpy (the loss of thickness versustime) can be calculated. One convenient formula forcalculating the corrosion rate, in mils per year, of

35、smallcorrosion test coupons can be found in the NationalAssociation of Corrosion Engineers (NACE) NACECorrosion Engineers Reference Book. The formula is:mpy = C weight loss K/(area time)where:C is 2,820Weight loss is measured in gramsK is a density factor (1.000 for carbon steel)Area of the sample i

36、s measured in square inchesTime is measured in daysThe above formula can be easily programmed on acomputer such that data entries for calculation needonly include the weight loss, specimen area, mate-rial, and time of exposure. Density factor chartsfrom the Reference Book can be preloaded into theco

37、mputer program, and the computer instructed toautomatically select the correct C values and den-sity factors for the various alloys needed to calcu-late the corrosion rate for each test. Alternate formsfor the formula are also given in the Reference bookto permit using different units for the exposu

38、retime, weight loss, and corrosion rate, but the appro-priate C and K values must also be incorporated.An example of the formula use follows:A carbon steel coupon with an exposed surface areaof 2.5 square inches was exposed for 20 days. Themeasured weight loss was 0.15 grams, and its den-sity factor

39、 is 1.0. The calculation is:mpy = (2,820 0.15 1.0)/(2.5 20) = 8.46 mpyAvoiding General CorrosionTo protect against general corrosion, the most obvi-ous solution is to avoid contact between the metaland the corroding environment. Keep bare steelparts in a controlled environment (low humidity,room tem

40、perature) and you will usually eliminatethe possibility of general corrosion. Simply stated,eliminate the corrosive environment from contactwith the metal and no corrosion occurs. Often, asyou may imagine, this is not the practical solution,so other methods had to be developed.For many alloys expose

41、d to uniform corrosion con-ditions, one approach is to add a “corrosion allow-ance.” In practice, this means making the itemthicker than that required by the engineering de-sign. Instead of using 1/4 inch steel plate when theknown general corrosion rate is 10 mpy, select aplate thickness of 3/8 inch

42、, and the extra 1/8 inchof steel thickness will withstand an additional 6years of service if corrosion occurs on both sides. Ifcorrosion will only occur on one side, an additional12 years of life is gained. Corrosion allowances areoften a very effective and economical approach formany alloys and con

43、ditions of general corrosion.Another common method to reduce atmosphericcorrosion of steel is to apply a protective coating tothe metal surface to exclude the corroding environ-ment from the metal surface; a coat of paint properlyapplied will provide excellent protection for anextended period of tim

44、e. A protective coating oftenapplied to steel to improve resistance to atmosphericexposure is zinc. Zinc coatings, also called “galva-nizing,” can be applied to the steel surface by severaldifferent methods. Dipping fabricated steel struc-tures into molten zinc is a common method, and iscalled “hot

45、dip galvanizing.” Zinc coatings can alsobe applied by electroplating or spray painting. Thezinc not only acts to exclude the environment fromthe metal surface, but can also protect carbon steelthrough a galvanic reaction even if the zinc coatingis damaged and incomplete. The zinc will prefer-entiall

46、y corrode, and the steel base metal is pro-tected. Galvanic protection will be discussed ingreater detail under the topic of Galvanic Corrosion.There are other methods for controlling general cor-rosion and they include lining the metal that willcorrode with a protective one, adding corrosioninhibit

47、ors to the corroding environment, anodic orcathodic protection of the component, or upgradesto other metals having better corrosion resistance.(Anodic protection refers to the practice of usingsacrificial anodes to protect the structure, and ca-thodic protection refers to the use of impressed cur-re

48、nt for the same purpose.) Of all the various formsof corrosion, the corrosion engineer usually prefersto deal with general corrosion because its effects aremore often well known and usually more predict-able. General corrosion usually results in a long-term condition prior to failure, and its avoida

49、nce isusually straightforward. It does, however, consti-tute a major portion of the economic loss sufferedby industry, and its detriment cannot be minimized.6 AWS Practical Reference GuideCorrosion of WeldsCauses and CuresPitting CorrosionPitting corrosion differs from general corrosion pri-marily in that pitting corrosion damage is quite lo-calized rather than covering the entire surface. It isdefined as “Extremely localized corrosion, resultingeventually in holes in the metal.” The total area basedon the pits diameters is usually quite small com