Waukesha Cherry-Burrell Products: FittingsCorrosion of Stainless Steel Stainless steel has been used for about a half century in the manufacturing of dairy and food processing equipment. In the majority of environments encountered in these industries, Stainless alloys provide high strength, ease of fabrication, attractive appearance and good corrosion resistance.Yet in spite of these virtues, stainless steel is not immune to corrosion. Of all the corrodents found in dairies and food plants, the vast majority of corrosion failures of stainless steel are attributable to environments containing chloride ions. This will become apparent in subsequent sections of this article. First, it should be pointed out that the types of stainless steels being considered are AISI types 304 and 316.
Both of these alloys exhibit corrosion-resistant characteristics by virtue of a thin protective chromium oxide film, or passivation layer which forms naturally when the metal surface is exposed to oxygen. With this layer intact, the metal is protected. But if the film is penetrated, damaged, or destroyed, the protection is lost and corrosion can occur very rapidly. The protective film is particularly susceptible to attack by the chloride ions.
Intergranular Corrosion Intergranular corrosion is caused more by welding techniques than by any other factors. Under a microscope, a steel surface consists of a patchwork of areas where the metal grain is oriented in different directions. When 300 series stainless steels (303, 304, 316 etc) are held in the range of 800° to 1650°F, chromium and carbon in the steel combine to form chromium carbide, which precipitates out, or accumulates at the grain boundaries. This depletes the adjacent areas of chromium, thereby reducing the resistance of those areas to corrosion. When welding, temperatures of the metal can reach 2900°F (1600°C), so weld heat control is important. Intergranular corrosion is characterized by corrosive attack in the base metal adjacent to a weld. It is typically found where field welds have been made such as in welded pipeline systems. The use of the low carbon grades such as 304L and 316L is one way to avoid the danger of carbide precipitation. However, it is more common and more economical to use care while welding by keeping the heat as low as practical and cooling the area quickly.
Crevice Corrosion As the name implies, this form of corrosion is an intense local attack within crevices or shielded areas exposed to corrosives, especially those containing chlorine or chlorides. Often, this is associated with bad gasket surfaces, lap joints or surface deposits. In plate heat exchangers, each of the hundreds of contact points- where one plate contacts the next for support, forms a crevice. Crevices in a processing system must be taken into consideration in setting up the processing/cleaning/sanitizing schedules. A major consideration is thorough cleaning during the CIP cycle to assure that all soil is removed completely. Residual soil can act as an artificial crevice and trap stagnant pools of hypochlorite solutions during sanitizing. When hypochlorite solutions are used for sanitizing:
Before introducing a hypochlorite sanitizer into a system, be sure the entire system is cooled to room temperature and all acids from the cleaning cycle have been removed by thorough rinsing.
Pitting Corrosion Pitting corrosion is a form of extremely local crevice corrosion and is usually caused by the presence of chlorides trapped in stagnant spots, which results in penetration of the passive film and corrosive attack of the steel. These stagnant spots can be the result of poor design, but more commonly they are caused by inadequate cleaning procedures. Residual soil traces can trap chlorides and become sites where pitting begins. The importance of thorough cleaning followed by draining and air-drying cannot be over emphasized in the care of stainless steel.
Stress Corrosion Cracking Stress corrosion cracking in austenitic stainless steel (like 304SS & 316SS) is caused by a combination of two conditions:
If exposure to chlorides cannot be avoided in a given application, consideration should be given to elimination of surface tensile stresses. Heat treatment is one method of eliminating stresses, but it is often impractical due to distortion or cost. Another method that has been used successfully is the conversion of the tensile stresses to compressive stresses by shot peening the surface. When these approaches are taken, operating temperatures should be kept as low as possible, and good cleaning procedures should be followed to avoid the formation of pits, which act as stress concentration points.
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