Castings intergranular corrosion

Fig. 19.19 Intergranular corrosion plot for a sensitised cast CF-S stainless steel (0 08 7o C max., 8-11% Ni, 18-21% Cr) in H2 SO4 at 40°C as a function of potential and concentration of acid (after France and Greene )...

Intergranular corrosion occurs in stainless steels and alloys based on nickel, aluminium, magnesium, copper and cast zinc. In the following sections we shall look at the three former groups in some detail. 

Cast alloys must be made from at least 99.99% purity zinc ingot to comply with the standards of the American Society for Testing and Materials (ASTM B6) and the International Organization for Standardization (ISO 752) a standard is in preparation by the Comit Europeen de Normalisation (CEN European Committee for Standardization). The harmful impurities, which occur naturally in zinc or as contaminants in the other alloying ingredients, are lead, tin, cadmium, indium, and thallium. These impurities are limited by specification higher contents than specified may make the alloys susceptible to intergranular corrosion, particularly when exposed to warm, moist environments. 

Fifty years ago, intergranular corrosion of the zinc—4% aluminum casting alloys was a problem because of high impurity levels, notably of lead, tin, and cadmium. The subsequent introduction of 99.99% zinc as the base for these casting alloys eliminated intergranular corrosion under normal service conditions. However, an inherent tendency for intergranular corrosion exists, and this defect can still occur if pure zinc-aluminum alloys are exposed to temperatures above about 70°C under wet or damp conditions. Research has... 

Metallographic examination revealed limited intergranular corrosion at the casting surface. The measured maximum depth of penetration was 100 p,m after 300 hours of testing. Intergranular corrosion usually progresses linearly with time and, therefore, a maximum depth of penetration of only 0.5 mm is expected after a service life of the car engine equivalent to 160,000 km. 

The general and intergranular corrosion of most of the alloys tested was reduced by copper additions, with the exception of alloys 13 and 17, which had very high combined tin and lead contents. Outdoor, 2 year exposures showed very similar results. In conclusion, Kehrer suggested reducing the allowable tin content of die castings to 10 ppm and the combined lead and cadmium level to 60 ppm. 

Raising the temperature increases die dissolution of zinc and the ZA alloys (Belisle and DuFresne, 1986) in water. A marked increase occurs up to about 60°C, followed by a decrease at higher temperatures. Intergranular corrosion of the zinc-aluminum casting alloys is a risk above about 70 C in wet or humid conditions (e.g., in steam), when no protective layer can form and selective dissolution of the structure occurs. A steam test at 95°C is indeed a standard method of checking whether alloys have been correctly produced. 

C. McCaul. 1991. Evaluation of intergranular corrosion susceptibility in as as-welded high alloy austenitic stainless steel casting, British Corrosion Journal, December. 

Intergranular corrosion. The only reference to intergranular attack in cast irons involves ammonium nitrate (NH4NO3), in which unalloyed cast irons are reported to be intergranularly attacked. 

I Various types of intergranular corrosion, (a) In- terdendritic corrosion in a cast stmcture. b) In-terfragmentary corrosion in a wrought, unrecrystallized... 

The ZA alloys have better general corrosion resistance than zinc or the zinc-4% aluminum alloys in many environments, while alloying and technological improvements increase the resistance to intergranular attack, which was historically a problem with zinc-4% aluminum castings produced from material in which impurities were inadequately controlled. These advances also allow more tolerance of impurities in specifications. 

---