7. Coating Failures

7.4 Types of Failures

3. Design Related Failures

(a) Failure at edges:

These failures are attributed to lack of proper design which causes poor adhesion of coatings. A good design excludes sharp edges and corners as it is difficult to coat these areas with even thickness. Edges are corrosion sensitive areas, particularly in marine environment.

Failure of coating at edges of bulkheads.

(b) Failures at corners

Corners represent areas of moisture entrapment and areas where differential aeration cells can be formed. Coating in the corners fails often by blistering due to adsorption of the moisture in the corners. Coating pulls out from the corners and corrosion is developed under the coating. The coating on the exterior of the corners is subjected to the same difficulty as the edges. The best remedy is to eliminate sharp corners from the design and to replace them with smooth corners.

Failure of coating at corners.

Another important point for a good coating application is the accessibility for coating. Figure below shows two examples of this aspect.

(c) Failures at welded areas

Welds represent the potential sites for failures because they produce discontinuities on the surface of the alloy and bring about metallurgical variation. Spot welds can lead to the development of undesirable crevices. Continuous welds are preferred over the intermittent welds as the latter do not completely block crevice formation and moisture traps.

Butt welds are preferable over the lap welds. Preferred design features for joints are shown in figure below. If insufficient coating is available, coat the cathodic area in preference to the anodic area.

Failure of coating at weld caused by too large root width.

Excessive gap.