Coating failure, causes

A reasonably close match of thermal expansion of the coating and substrate over a wide temperature range to limit failure caused by residual stresses is desired for coatings. Because temperature gradients cause stress even in a weU-matched system, the mechanical properties, strength, and ductUity of the coating as well as the interfacial strength must be considered. 

Major causes for coating failure are surface cracking and undetected pinholes or voids. These can be repaired and serious problems avoided. Coatings generally fail in different modes, these are chemical failure, abrasion failure, adhesive failure, cohesive failure and undercoat corrosion. For performance evaluation of coatings on experimental basis on these parameters various ASTM and BS specifications are presently being used. 

One form of film break-up has been mentioned previously, in which the surface of a relatively thick film becomes consolidated over a soft unconsolidated layer. This can lead to shear in the soft layer, especially with high non-conformal loading, and the consolidated layer will break away. Loss of adhesion and film break-up can also occur, especially with a bonded film, if the surface pre-treatment has been badly performed, so that the coating simply fails to adhere and breaks away. These cases may be considered as premature failures caused by poor film preparation. 

Figure 30. Large area failure caused Figure 31. Swelling cracks in a sulfur by thin area in sulfur coating coating of a wetted SCU granule...

The hydrophilic thermosetting materials do not usually satisfy well these two requirements. Water concentrates at their coating/substrate interface, causes corrosion, delamination, and coatings failure. As a result of these problems in the hydrophilic thermosets, there is a strong need for a hydrophobic material. 

Home Appliances This is one of the largest consumer products industries. The cost of corrosion in home appliances includes the cost of purchasing replacement appliances because of premature failures caused by corrosion. For water heaters alone, the replacement cost was estimated at 460 million per year, with at least 5% being corrosion related. The cost of internal corrosion protection for all appliances includes the use of sacrificial anodes ( 780 million per year), corrosion-resistant materials, and internal coatings. The annual cost of external corrosion protection with coatings was estimated at 260 million. The total direct corrosion costs are at least 1.5 billion. 

Thermal Expansion Matching. The coefficients of thermal expansion (CTE) of coating and substrate should match as closely as possible. The CTE of the coating is usually lower than that of a metallic substrate and, upon cooling from the deposition temperature, thermal stresses are produced which may cause cracks and delamination. Such considerations have led to the development of low-temperature deposition processes such plasma-CVD or metallo-organic CVD (MOCVD) which minimizes these stresses and reduces the chance of coating failure (see Sec. 3.1).P1... 

Coating failures result from two basic causes ... 

Other factors in the atmosphere that cause corrosion or degradation of the coating are UV light, temperature, oxygen, ozone, pollutants, and wind. Types of failures caused by these factors are ... 

Figure 14.2 Breakdown of the root causes behind 84 coating failures thoroughly investigated.

Table 14.1 Common Coating Failures, Their Causes, and Remedies [3]...

The first cause (flux interference) can be removed by adopting a proper cleaning process. Welds are the potential holidays in underground pipelines. Weld areas are generally the first regions to show coating failures in a non-aqueous environment. 

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