Intergranular attack

Localized deterioration Corrosion (especially pitting and intergranular attack), erosion, cavitation, mechanical wear, and so on (see Case History 9.8). 

A somewhat similar phenomenon is knife-line attack which may be observed after welding titanium or niobium stabilised austenitic stainless steels. In this case there is a very narrow band of severe intergranular attack along the interface between the parent metal and the fusion zone. During welding, the parent metal immediately adjacent to the fusion zone is heated to just below the melting point and both chromium carbides and niobium or titanium carbides dissolve completely. On cooling rapidly, the conditions are such that when relatively thin sections are welded, neither chromium carbide nor niobium or titanium carbide have time to precipitate. If the weld is now... 

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

Coriou, M., Grail, L, Mahieu, C. and Pelas, M., Sensitivity to Stress Corrosion and Intergranular Attack of High-nickel Austenitic Alloys , Corrosion, 22, 280 (1966)... 

Ward, C. T., Mathis, D. L. and Staehle, R. W., Intergranular Attack of Sensitised Austenitic Stainless Steel by Water Containing Fluoride Ions , Corrosion, 25, 394 (1969)... 

Ijzermans, A. B., Pitting Corrosion and Intergranular Attack of Austenitic Cr-Ni Stainless Steels in NaSCN , Corros. Sci., 10, 607 (1970)... 

Stress below the proof stress does not normally affect corrosion rates. Cyclic stresses in combination with a corrosive environment (corrosion fatigue) can produce failure at below the ordinary fatigue limit. Alloys susceptible to intergranular attack may corrode faster when stressed (see Section 8.5). 

Investigations into the effects of arsenic and phosphorus in single-phase brasses on their susceptibility to intergranular attack and stress-corrosion cracking in seawater have shown that the normal addition of arsenic to... 

At temperatures above 300°C, low-carbon nickel (0-02% C) is preferred to avoid the possibility of intergranular attack developing after long exposure if material of higher carbon is employed it should be annealed after fabrication and before exposure to caustic alkalis to prevent stress-assisted intergranular corrosion. 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

Bismuth Liquid bismuth has little action on tantalum at temperatures below 1000°C " , the rate of attack at 870°C being less than 0.13 mm/y, and exerts not detrimental effects on the stress rupture properties of tantalum at 815°C, but is causes some intergranular attack at 1000°C . 

Despite their improved resistance to general corrosion by sulphur, the dilute alloys with many elements are sensitive to intergranular attack and embrittlement, and, at temperatures above 635°C, to eutectic formation. 

Nickel and nickel-rich alloys must be considered as having generally poor resistance to molten metals. Eldred has made a systematic investigation of the attack of liquid metals on solid metals and alloys, and his results for nickel, and nickel-chromium and nickel-copper alloys, are summarised in Table 7.35. These are for tests at up to 500 C and apart from potassium and sodium all the low-melting-point metals investigated produced moderate to severe attack on the nickel-rich materials. Furthermore, the values for many of the combinations given in the table indicate a marked tendency to preferential intergranular attack. 

Standard Recommended Practices for Detecting Susceptibility to Intergranular Attack in Stainless Steels, ANSI/ASTM A262 79 (1979)... 

For both heat-affected zone corrosion (intergranular attack) and knifeline attack the heat flux during welding and the time at temperature can critically affect the severity of the attack. Both these factors may vary from one welder to another, and when preparing pieces for corrosion testing not only should fabrication welding conditions be accurately reproduced, but the work of more than one welder should be evaluated. 

This test has been developed and used by Streicher as a screening test to be used in conjunction with the tedious boiling nitric acid test for assessing the susceptibility of stainless steels to intergranular attack as specified in ASTM A262 1986, and will be considered subsequently in the section concerned with intergranular attack of Cr-Ni-Fe alloys. 

Intergranular corrosion of Fe-Ni-Cr alloys has been the subject of a comprehensive review by Cowan and Tedmon who summarised the various tests used for determining susceptibility (Table 19.3). Of these tests. Nos. 1-5, which are regarded as reliable test procedures by the ASTM, have been incorporated into ASTM A262 1986 Recommended Practice for Detecting Susceptibility to Intergranular Attack in Stainless Steel as follows ... 

Table 19.4 Maximum acceptable evaluation test rates specified by Du Pont for services where susceptible material would be intergranularly attacked ...

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