From a corrosion standpoint, the environments likely to be encountered in the food industry that may cause premature equipment failure may be classified under main headings:
Noncorrosive Mildly corrosive Highly corrosive Service fluids Alkaline detergents Acidic detergents Sanitizing agents
In general terms, natural foodstuffs such as milk, cream, natural fruit juices, and whole egg do not cause corrosion
stress corrosion cracking. Therefore, this form of damage is frequently also present (Fig. 20).
Depending on the method used to make the casting, the surface of the castings can be chemically modified, which reduces the corrosion resistance. Small components are usually cast by either the shell molding process or produced as investment castings. In the shell molding process, the sand forming the mold is bonded together with an organic resin that carbonizes when the hot metal is poured. This results in the metal adjacent to the mold having an enhanced carbon level with the formation of intergranular carbide precipitates and hence, a susceptibility to intergranular attack and other forms such as crevice, pitting, and stress corrosion cracking. Methods of overcoming this include solution annealing or machining off the carburized skin of metal.
With investment castings, the mold is made of zircon sand (zirconium silicate), and fired at a high temperature to remove all traces of organic material and wax that is used as a core in the moldmaking process. They do not, therefore, have this carburized layer and offer a much better resistance to surface corrosion.
Free-Machining Stainless Steels. Stainless steels are notoriously difficult to machine, especially turning, not so much because they are hard, but because the swarf tends to form as continuous lengths that clog the machine and to weld to the tip of the machine tools. One method of overcoming this is to incorporate a small amount, typically 0.2%, of sulfur or selenium in the alloy. These elements react with the manganese to form manganese sulfide or selenide, which, being insoluble in the steel, forms as discrete pools in castings or as elongated, continuous stringers in, say, wrought bar. The effect of the sulfide inclusions is to cause the material to form chips rather than long strings of swarf when being machined. Both manganese sulfide and selenide have virtually zero corrosion resistance to dilute mineral acids or other corrosive media. Thus, the free-cutting variants have a much lower corrosion resistance than their designation would imply. Indeed, some believe that the addition of sulfur to a type 316 material will offset the beneficial effect of the alloying addition of molybdenum. As stated, the sulfide inclusions will occur in castings as discrete pools, and therefore there will
not be a continuous corrosion path. However, one of the products of corrosion in acidic media will be hydrogen sulfide, which has a profound effect on the corrosivity of even dilute mineral acids, causing attack of the austenitic matrix.
In the case of wrought materials, in particular bar stock, the sulfide inclusions are present as semicontinuous stringers and can suffer so-called end-grain attack in mildly corrosive media.
Stainless-steel nuts and bolts, which are produced on automatic thread-cutting machines, are invariably made from free-cutting materials. Figure 21 illustrates the difference in corrosion resistance of a bolt made from this and a nonfree machining 316. Both bolts were exposed to the same mildly acidic environment.
When specifying materials of construction, quite clearly due cognizance of this difference must be recognized. Any components turned from bar stock which are likely to come into contact with potentially corrosive environments should always be specified in 316 and not in the free machining, sulfur-containing variant.
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