They are used for low friction cases and high mechanical loads appliances. Jewel bearings are normally small and are applicable for work that needs to be very precise carrying low loads as is the case with clocks. Fluid bearings have two edge seal held faces separated by a fluid. They have no friction and require no maintenance. They are used where low friction is needed for heavy loading. In magnetic bearings, magnets are used to act as separation of the faces. They don’t suffer friction and require no maintenance.
However, eddy currents may lead to inconsequential movements ( Lu, Berzins, Goodyer & Jimack, 2005). Figure 1 is a representation of a ball bearing. Figure: Ball Bearing In the course of operation, bearings are subjected to heavy mechanical loads, high speeds and play during load application and friction among others (Khonsari & Booser, 2001). All these cause the bearing mechanical properties to depreciate and the surface to wear leading to exposure to the environment. When the surface of the bearing is exposed, it may be corroded.
Corrosion in bearings could be as a result of condensation of moisture as the temperature changes or wear of the seals that could allow water leakage inside the bearing (Ahmad & Institution of Chemical Engineers, 2006). Corrosion of bearing surfaces can also occur when the bearing is not fully dried after washing. According to Harnoy (2003), corrosion refers to degradation of material properties when it chemically reacts to its environment. In bearings, this chemical attack may occur over a wide area or it can be linear.
For instance, it can attack the line at which a race and roller of stationary bearings come into contact. There is various classification of corrosion which could cause damage to bearings depending on environment, corrosion damage morphology or the material itself. These include general, pitting, crevice, Inter-granular, de-alloying and erosion corrosion modes. Corrosion can also occur as a result of fracture induced by the environment. In general corrosion, electrochemical or chemical reactions eat away the exposed surface of a material proceed continuously and at a constant rate.
Consequently, the exposed metal loses its thickness and surface appearance is changed (Idaho National Laboratory, 2005). By reducing the thickness of the material and the decrease in mechanical strength, the bearing may fail and in severe cases, leaking is experienced. Pitting corrosion on the other hand involves high corrosion rates concentrated on a spot within the material surface. Mostly, it is prevalent where a coating which would otherwise hinder the inner material from corroding has flawed or in metallic materials that combat corrosion by forming a hydroxide, native oxide or a film of salt.
In environment rich with halide ions, pitting is a common phenomenon because these ions either prevent reformation of film coatings or stimulates their breakdown. Pitting initiation time also relies on the temperature magnitudes with its rate increasing at heightened temperatures. For instance, there are cases where a bearing is placed in a region with high operating temperatures or where friction is high. Once pitting has began, it proceeds at an exponential rate that is higher on the underlying material that on the exposed one (Idaho National Laboratory, 2005).
Pitting is responsible for cracks that enhance material failure as well as perforations found in storage bottles among others. In Jones, In Haggard & In Greenwald (2014) describes the other type of corrosion as the crevice corrosion. An exposed material on the bearing surface such as occluded regions which do not freely mix with the environment, some of these regions could be the crevices. This leads to a difference in the interaction of these regions with the chemical environmental bulk that may result into corrosion within these occluded regions.
When alloys are exposed to environments containing halide ions such as chloride ions or when the material is a passive metal, there are high chances that it will undergo crevice corrosion.
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