Machinery's Handbook, 31st Edition
554 Corrosion loading and introducing surface compressive stress through methods such as shot peen - ing, laser peening, or grit blasting. Tribocorrosion.—When a material degrades due to a combination of erosion/wear and corrosion, the process is called tribocorrosion. Mechanical wear often accelerates the corrosion process; in turn, corrosion attack on the material often accelerates wear. There are two commonly encountered subtypes of tribocorrosion: Fretting Corrosion: This occurs when two parts, exposed to a corrosive medium, rub forcibly against each other in a repetitive manner. The movements typically are very small; vibration is a common culprit. Contact surfaces experience mechanical wear and corrosive attack simultaneously, often leading to crack formation and fatigue failure. Bolted joints are especially vulnerable, as are bearings that undergo oscillating move - ments. Mitigation methods include eliminating movement between parts (particularly small oscillating movements), employing protective lubricants or sealants, adjusting hardness of one or both parts, and reducing contact loading, if possible. Erosion Corrosion: Parts exposed to a flowing corrosive medium may exhibit local - ized areas where mechanical erosion occurs and corrosion is intensified. Optimizing flow geometry and velocity can help reduce turbulence and prevent such cavitation. Other methods include filtering out abrasive particles in the fluid, smoothing surfaces to remove imperfections, and minimizing edges. Hydrogen Embrittlement.—High-strength steels, titanium alloys, and aluminum alloys are particularly susceptible to this type of attack. When hydrogen is absorbed into the material, this can cause embrittlement and cracking along grain boundaries, posing a risk of sudden failure. Processes that can cause excess hydrogen exposure include arc weld- ing (unless low-hydrogen electrodes are used), electroplating, pickling, and phosphating. Mitigation steps include limiting hydrogen exposure, baking off hydrogen after exposure, relieving residual stresses, using barrier coatings, and substituting less vulnerable materi - als. Note that cathodic protection accelerates hydrogen embrittlement, especially in acid environments, so it should be used with caution when working with susceptible materials and processes. Corrosion Fatigue.—Fatigue failure can occur prematurely in parts exposed to a cor- rosive environment while experiencing cyclic or alternating loading conditions. Cracks formed in this process usually are nonbranching and transgranular, form early, and grow at an accelerated rate, leading to substantially reduced fatigue life of parts. Prevention includes reducing loads and stresses, as well as normal corrosion mitigation methods.
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