How many engineers know that corrosion is generally regarded as the prime cause of failures? In 1986 Batelle Labs estimated the annual cost at that time of $186 billion dollars—4.2 percent of GNP. Gears, roller bearings, and cams all corrode from the same sources and are not immune to this problem that is now receiving its due amount of attention.
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Corrosion is a natural process of transformation. With few exceptions it is created by minute electric currents, chemical reaction, and except for high temperature metal gas reactions, moisture (water). The word corrosion is from the Latin “corrodere,” meaning to gnaw away. Minor variations in operating conditions can influence the rate of corrosion. For example, the atmospheric corrosion rate in mils per annum for rural is 0.1 to 3, industrial 7 to 15, and marine 4 to 24. Atmospheric corrosion also reduces the fatigue life of harder steels at a greater rate than less hard materials. Nitrided steels have a higher resistance to some types of corrosion. Finely finished surfaces corrode less easily and stain, rough surfaces corrode more easily leading to pitting. Fatigue life is adversely affected in both metal and plastic gears. The latter experience thermal deterioration, suffering from incompatible lubricants and aggressive atmospheres. Seals are also affected by age hardening. An even more adverse affect occurs when corrosion is accompanied by fatigue loads. The corrosion attack roughens the surface and produces stress raisers that will initiate fatigue cracks. The tooth surface may appear stained, or even show red rust deposits. Rust particles are hard and can lead to abrasive wear. Beneath these visual deposits rough irregular pits may be present. The entire tooth surface grain boundaries are usually under attack. The sources of the corrosion can be lubricant contamination, an anti-scuff additive, acid etch following a grinding crack inspection, or even the touch of a human hand.
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Enclosed gear drives can be especially prone during operation and storage because temperature changes and the humidity in the air results in condensation within the housing. A specific type of corrosion is termed “fretting” corrosion. The corrosion is affected by minute vibrationary motions such as occurs during shipping. Cyclic loading, reverse-bending, torque transmission, expansion and contraction, and transient vibration under load can lead to fretting. The contacting surfaces are pressed together, causing a metal to metal contact and subsequent adhesion of the surface asperites. These asperites break from the relative motion and leave an abrasive residue of iron oxide powder the color of cocoa. Hydrogen embrittlement occurs when certain steel alloys absorb excessive amounts of hydrogen. The blame is often—and usually incorrectly—charged to the heat treatment and almost always confined to gears with a hardness of 40 Rockwell C. It can occur in assembly or service, and cadmium or electrio plating, when not followed by baking, is a prime source of the embrittlement. If the steel has a flaw the hydrogen gravitates to the area of stress concentration and initiates cracking. The crack propagates through the weakened grain boundaries and leads to a fracture. Stress-corrosion cracking (SCC) occurs when cracking is accelerated by the combined affects of stress and corrosive action. The stresses can include residual stress from the fabrication or heat treatment, or a combination of residual and operating stresses. The condition is initiated at several locations by localized pitting and/or intergranular attack. It continues slowly until a crack develops. Typically there is an incubation period when corrosion pits nucleate and grow until cracks are initiated. A microscopic examination is required to confirm that it was the result of SCC. Plastic gears are adversely affected by the moisture level, which can cause them to expand; the reverse is true when the moisture level is low. The problem is compounded when more stable metal components are attached to the plastic gear. Some plastics such as Nylon 6/6 become more brittle when dry than at elevated moisture levels.
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Environmental stress-cracking (ESC) is a brittle fracture mechanism, and the leading cause of plastic component failure. Estimates made in 2006 suggested that 25 percent of failures were due to ESC. The plastic resin is embrittled by a chemical agent while under a tensile load. The chemical permeates the molecular network and interferes with the binding of the polymer chains. Increased crystallinity improves ESC resistance. The temperature affects the rate of chemical diffusion and subsequent crack propagation. ESC curves have been developed by the material suppliers for many plastics. A chemical method, stain resistance, solvent stress-cracking resistance, and environmental stress cracking resistance (ESCR) tests are being employed to analyze plastic failures.
