Corrosion grading scale

Table 9.1 Grading scale used to evaluate the degree of interface and surface corrosion Thomas et ai, 1988)...

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

The reaction is frequently carried out in the presence of scrap iron (with low P and S content) to produce ferrosilicon alloys these are used in the metallurgical industry to deoxidize steel, to manufacture high-Si corrosion-resistant Fe, and Si/steel laminations for electric motors. The scale of operations can be gauged from the 1980 world production figures which were in excess of 5 megatonnes. Consumption of high purity (semiconductor grade) Si leapt from less than 10 tonnes in 1955 to 2800 tonnes in 1980. 

One of the most serious corrosion problems associated with type 316 stainless steel is its susceptibility to crevice corrosion. The incidence and extent of this type of corrosion in surgical implants was stressed by Scales eta/. who reported the presence of crevice corrosion in 24% of type 316L bone plates and screws examined after removal from patients. This record however compared favourably with the presence of crevice corrosion in 51 % of 18-8 stainless plates, demonstrating the superiority of the molybdenum-containing grade. 

As dealt with in previous ehapters, the eorrosion resistanee of stainless steel is due to passivation by a surface film of chromium oxide. The chromium content is higher than about 11%, and the low-temperature corrosion resistance as well as the resistance to oxidation and mill scale formation at high temperature increase with increasing content of chromium. Pure chromium steels are either ferritic (low C-content, non-hardenable by heat treatment) or martensitic (traditionally higher C-content for most grades, hardenable by heat treatment). Wifli sufficient content of Ni the structure becomes austenitic, which gives increased formability, weldability. 

The best known type of petrolatum is the "petroleum jelly" used in ointments. There are also heavier grades for industrial applications, such as corrosion preventives, carbon paper, and butcher s wrap. Traditionally, the terms slack wax, and refined wax were used to indicate limitations on oil content. Today, these classifications are less exact in their meanings, especially in the distinction between slack wax and scale wax. 

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