Corrosion intergranular

Intergranular corrosion is a form of localized attack in which a narrow path is corroded out preferentially along the grain boundaries of a metal. It often initiates on the surface and proceeds by local cell action in the immediate vicinity of a grain boimdary. Although the detailed mechanism of intergranular corrosion varies with each metal system. 

The constituent may be anodic, cathodic, or neutral to the base metal or adjacent zone. Examples of anodic constituents are the intermetallic phases Mg Al and MgZn in aluminum alloys and Fe N in iron alloys. Examples of cathodic constituents are FeAl3 and CuAlj in aluminum alloys and Fe C in iron alloys. Examples of neutral constituents are Mg Si and MnAl in aluminum alloys and Mo C and WjC in wrought Ni-Cr-Mo alloys. 

It is generally noted that the grain boundaries are particularly reactive. This is the reason for higher corrosion at grain boundaries, in certain cases, rather than at the grain surfaces. 

1 Austenitic Stainless Steels. For austenitic alloys, the sensitizing temperature range is approximately 425-875 C (800-1610°F). The degree of 

2 Theory and Remedies. Since intergranular corrosion of austenitic 

If the alloy is rapidly cooled through the sensitizing zone, carbon does not have time either to reach the grain boundaries or to react with chromium if some carbon is already concentrated at the grain boundaries. On the other hand, if the 

Microprobe scans of sensitized stainless steels have indicated chromium depletion and nickel enrichment at grain boimdaries [21]. Radioactive C introduced into an austenitic 18% Cr, 12.8% Ni, 0.12% C stainless steel has demon- 

There are at least three effective means for avoiding susceptibility to intergranular corrosion  

Effect of carbon content on carbide precipitation. Carbide precipitation forms in the areas to the right of the various carbon-content curves. 

Intergranular corrosion depends upon the magnitude of the sensitized material exposed and the aggressiveness of the envirorunent to which the sensitized material is exposed. Many envirorunents do not cause intergranular corrosion in sensitized austenitic stainless steels. 

Titaruum and niobium additions equal to five or ten times the carbon content, respectively, permit the carbon to precipitate as titanium or niobium carbides during a sensitizing heat treatment. The carbon precipitation does not reduce the chromium content of the grain boundaries. 

Intergranular attack may also occur due to mechanisms other than carbide precipitation. The ferrite phase, if present, may be selectively attacked by reducing acids, such as hydrochloric or sulfuric. Its thermal conversion product, the sigma phase, is selectively attacked by oxidizing acids such as nitric. 

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Cihal V 1984 Intergranular Corrosion of Steels and Alloys (Amsterdam Elsevier)... 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Copper increases tensile strength and hardness and offers some protection against elements that promote intergranular corrosion. However, copper reduces impact strength and dimensional stabiUty owing to aging and is therefore kept at 1.25% max. 

Intergranular Corrosion Selec tive corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

Evaluation of Results After the specimens have been reweighed, they should be examined carefully. LocaHzed attack such as pits, crevice corrosion, stress-acceleratedcorrosion, crackiug, or intergranular corrosion should be measured for depth and area affected. 

Hastelloy C-4 is almost totally immune to selective intergranular corrosion in weld-heat-affected zones with high temperature stabihty in the 650-I040 C (I200-I900 F) range Hastelloy C-22 has better overall corrosion resistance and versatihty than either C-4 or C-276 (in most environments). 

The orientation of the cracks reveals that cyclic bending stresses or cyclic axial stresses were active. The intensification of these stresses at pits and intergranular corrosion sites produced the cracks observed. 

Intergranular corrosion-fatigue cracks in copper may he difficult to differentiate from stress-corrosion cracking. The longitudinal orientation of the cracks revealed that the cyclic stresses were induced by fluctuations in internal pressure. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

Intergranular corrosion depends on the length of time the steel is exposed to the sensitizing temperature (500-750°C), even if made from low-carbon or titanium-or niobium-stabilized steel. 

This alloy has a nominal composition of 65% nickel, 28% molybdenum and 6% iron. It is generally used in reducing conditions. It is intended to work in very severely corrosive situations after post-weld heat treatment to prevent intergranular corrosion. These alloys have outstanding resistance to all concentrations of hydrochloric acid up to boiling-point temperatures and in boiling sulfuric acid solutions up to 60% concentration. 

The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy. 

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