Coatings continued service tests

Eor steel and other ferromagnetic materials, property deterrnination is more difficult. Other tests are made to measure the continuity of protective metallic coatings. Residual stresses induced in welded stmctures and in components in service owing to chemical attack may contribute to early failure. 

In practice there is a continuous range of coating hardnesses varying from very soft to very hard, and the hardness is determined not only by the binder but by the concentrations of molybdenum disulphide and other solid components. The definition of hardness is also relative to the service contact loads. A coating of intermediate hardness may behave like a hard film under low contact pressures or like a soft film under high contact pressures. As a result, it is very difficult to compare the quoted load-carrying capacity and wear lives of different films tested under different conditions by different research groups. 

Metal coatings are intended in most cases as corrosion protection barriers. In accelerated tests, the time required for corrosion should be reduced preferably by exposing the specimens continuously to conditions, which in service would arise only intermittently. At the same time, the conditions used should cause no damage of types other than that which would arise under conditions of natural exposure. 

In spite of serious shortcomings in providing guidance for service-life prediction, the continuous salt-spray test, which was originally designed to test coatings on metals, has been widely used to evaluate the resistance of metals to corrosion in marine service or on exposed shore locations [25 26]. Extensive experience has shown that, although salt-spray tests yield results somewhat similar to those in marine... 

---