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vicinity of the weld, making it difficult to spot and increasing the corrosion speed. Structures made of such steels have to be heated in a whole to about 1065 °C (1950 °F), when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important, as the carbon that would otherwise pose risk of formation of chromium carbide is already sequestered as niobium carbide.
308:, and the sensitized areas show as wide, dark lines where the etching fluid has caused corrosion. The dark lines consist of carbides and corrosion products. Intergranular corrosion is generally considered to be caused by the segregation of impurities at the grain boundaries or by enrichment or depletion of one of the alloying elements in the grain boundary areas. Thus in certain
420:. Such a stabilized titanium-bearing austenitic chromium-nickel-copper stainless steel is shown in U.S. Pat. No. 3,562,781. Or the stainless steel may initially be reduced in carbon content below 0.03 percent so that insufficient carbon is provided for carbide formation. These techniques are expensive and only partially effective since sensitization may occur with time. The
366:, when these steels are sensitized by being heated in the temperature range of about 520 °C to 800 °C, depletion of chromium in the grain boundary region occurs, resulting in susceptibility to intergranular corrosion. Such sensitization of austenitic stainless steels can readily occur because of temperature service requirements, as in
393:-annealing or solution-quenching, has been used. The alloy is heated to a temperature of about 1,060 °C to 1,120 °C and then water quenched. This method is generally unsuitable for treating large assemblies, and also ineffective where welding is subsequently used for making repairs or for attaching other structures.
113:). Around 12% chromium is minimally required to ensure passivation, a mechanism by which an ultra thin invisible film, known as passive film, forms on the surface of stainless steels. This passive film protects the metal from corrosive environments. The self-healing property of the passive film make the steel stainless.
172:
it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation. Another possibility is to keep the welded parts thin enough so that, upon cooling, the metal dissipates heat too quickly for chromium carbide to precipitate. The ASTM A923, ASTM A262, and
188:
dissolve in steel at very high temperatures. At some cooling regimes (depending on the rate of cooling), niobium carbide does not precipitate and the steel then behaves like unstabilized steel, forming chromium carbide instead. This affects only a thin zone several millimeters wide in the very
292:
Certain alloys when exposed to a temperature characterized as a sensitizing temperature become particularly susceptible to intergranular corrosion. In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive and intergranular corrosion results. This is
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other similar tests are often used to determine when stainless steels are susceptible to intergranular corrosion. The tests require etching with chemicals that reveal the presence of intermetallic particles, sometimes combined with Charpy V-Notch and other mechanical testing.
207:, where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure.
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refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the steel or alloy to be susceptible to intergranular corrosion or intergranular stress corrosion cracking.
304:
The photos show the typical microstructure of a normalized (unsensitized) type 304 stainless steel and a heavily sensitized steel. The samples have been polished and etched before taking the
332:-type alloys (Al-Cu) which depend upon precipitated phases for strengthening are susceptible to intergranular corrosion following sensitization at temperatures of about 120 °C.
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tearing the material apart. Similar effect leads to formation of lamellae in stainless steels, due to the difference of thermal expansion of the oxides and the metal.
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between the aluminium-rich crystals. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo
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are also susceptible to intergranular corrosion following sensitization in the temperature range of 420 °C–850 °C. In the case of the
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at the grain boundaries, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (this process is called
128:. This condition happens when the material is heated to temperatures around 700 °C for too long a time, and often occurs during
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in the steel and in case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000 °C and
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Several methods have been used to control or minimize the intergranular corrosion of susceptible alloys, particularly of the
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have been shown to segregate in the grain boundaries and cause intergranular corrosion. Also, it has been shown that the
203:
184:). Knifeline attack impacts steels stabilized by niobium, such as 347 stainless steel. Titanium, niobium, and their
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This situation can happen in otherwise corrosion-resistant alloys, when the grain boundaries are depleted, known as
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or heavy working leads to formation of long, flat grains, are especially prone to intergranular corrosion.
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Copper-based alloys become sensitive when depletion of copper content in the grain boundaries occurs.
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197:-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as
416:; carbide formation with these elements reduces the carbon available in the alloy for formation of
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is about seven times higher than the volume of original metal, leading to formation of internal
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Another control technique for preventing intergranular corrosion involves incorporating strong
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of the grains themselves. The alloy disintegrates (grains fall out) and/or loses its strength.
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34:
Microscope view of a polished cross section of a material attacked by intergranular corrosion
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increases together with its nickel content. A broader term for this class of corrosion is
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136:. When zones of such material form due to welding, the resulting corrosion is termed
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261:. Inter granular corrosion can be detected by ultrasonic and eddy current methods.
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Intergranular corrosion is a concern especially for alloys with high content of
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is higher at the grain boundaries and subject to such corrosion. High-strength
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is added for corrosion resistance, the mechanism involved is precipitation of
90:, of the corrosion-inhibiting elements such as chromium by some mechanism. In
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Other kinds of alloys can undergo exfoliation as well; the sensitivity of
140:. Stainless steels can be stabilized against this behavior by addition of
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in the stainless steels. Such elements have a much greater affinity for
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of the material are more susceptible to corrosion than their insides. (
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Intergranular corrosion induced by environmental stresses is termed
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ASTM A923 Intergranular
Corrosion Testing of Duplex Stainless Steel
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When crystallite boundaries are more corrosive than their interiors
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preferentially to chromium carbide, by lowering the content of
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also frequently exhibit lower strengths at high temperatures.
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alloys containing aluminum exhibit intergranular corrosion by
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ASTM A262 Intergranular
Corrosion Testing of Stainless Steels
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Another related kind of intergranular corrosion is termed
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are susceptible to lamellar corrosion, as the volume of
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characterized by a localized attack at and adjacent to
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often involves grain boundary depletion mechanisms.
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381:. For example, a high-temperature solution
355:in a marine atmosphere. Cr-Mn and Cr-Mn-Ni
400:formers or stabilizing elements such as
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204:exfoliation corrosion (metallurgy)
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344:800 show similar susceptibility.
288:Heavily sensitized microstructure
370:, or as a result of subsequent
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120:These zones also act as local
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1:
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385:, commonly termed solution-
379:austenitic stainless steels
280:Unsensitized microstructure
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74:transgranular corrosion.)
374:of the formed structure.
259:stress corrosion cracking
86:grain boundary depletion
297:with relatively little
44:intergranular corrosion
434:Intergranular fracture
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336:-rich alloys such as
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52:intergranular attack
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265:Sensitization effect
18:Sensitization effect
328:alloys such as the
312:, small amounts of
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223:lamellar corrosion
126:galvanic corrosion
115:Selective leaching
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422:low-carbon steels
418:chromium carbides
50:), also known as
40:materials science
16:(Redirected from
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466:
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368:steam generators
364:stainless steels
310:aluminium alloys
295:grain boundaries
235:tensile stresses
178:knifeline attack
162:tantalum carbide
154:titanium carbide
124:, causing local
122:galvanic couples
107:chromium carbide
99:stainless steels
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58:), is a form of
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132:or an improper
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225:. Alloys of
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68:crystallites
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246:Anisotropic
231:iron oxides
219:cupronickel
94:alloys and
78:Description
440:References
412:than does
361:austenitic
138:weld decay
96:austenitic
64:boundaries
62:where the
482:Corrosion
387:annealing
330:Duralumin
326:aluminium
299:corrosion
250:extrusion
195:Aluminium
170:quenching
60:corrosion
476:Category
428:See also
414:chromium
406:titanium
346:Die-cast
340:600 and
186:carbides
150:tantalum
142:titanium
103:chromium
101:, where
402:niobium
398:carbide
372:welding
342:Incoloy
338:Inconel
146:niobium
130:welding
410:carbon
391:quench
357:steels
334:Nickel
306:photos
212:copper
199:anodes
166:carbon
92:nickel
353:steam
322:brass
148:, or
349:zinc
318:zinc
314:iron
227:iron
160:and
404:or
182:KLA
72:Cf.
66:of
56:IGA
48:IGC
38:In
478::
389:,
214:.
156:,
144:,
42:,
180:(
54:(
46:(
20:)
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