Maintenance

Current system performance

When maintenance of steel structures is under consideration a key point to check is performance of the existing coating system. Deterioration is rarely uniform so it is essential to examine the overall performance and then to determine if the original specification was sound and adequate and why any breakdowns have occurred. Time spent in appraising these factors is generally well spent.

Surface inspection

If breakdowns have occurred check to find out whether any of these factors are involved:

Design deficiencies

These could include water traps causing puddling. Look at the drainage arrangements and make any necessary changes. If changes or modifications are impractical it may be necessary to upgrade the coating system.

Thin Films

If 'rash' or pinpoint corrosion is observed a likely reason is that the coating thickness was inadequate to protect the surface profile. One contributing cause could be the lack of accessibility to some areas and therefore the coating was not correctly applied or checked in those areas. Similar failures occur on sharp edges where film thickness is inadequate. Specify an initial extra coat on all such edges in the maintenance specification and ensure that adequate film thickness is specified for the areas where breakdown has occurred. Put the effort and money into the areas of breakdown and avoid the 'patch prime and one coat overall' approach. Use the sound areas as a guide to the dry-film thickness required.

Inadequate preparation

Generally the results can be seen in film eruptions and rust blisters breaking out at weak points due to dirt in the film brush marks and other areas of reduced film thickness. Clearly the surface of the metal was contaminated resulting in the continued corrosion process producing increased corrosion products beneath the film.

Environment

The coating is subject to the effects of wetting rain UV light heating cooling and chemical attack all of which weaken the film. During its life cycle a coating normally undergoes this sequence of events; loss of gloss discolouration small microscopic crazing of the film and larger line cracking. Ideally maintenance should be considered before the last defect is apparent. More severe failures are associated with the destruction of the coating by chemical attack and manifest themselves as saponification (total softening of the film) or removal in the manner associated with a paint remover. If these problems are encountered check if the system was wrong for the environment. Alternatively notice whether parts of the structure are subject to micro-environments which affect small sections only. Such a micro-environment is generally associated with chemical attack. These localised areas of corrosion are often found at the base of stanchions or in areas where there are water traps causing immersion conditions. Rapid deterioration of a previously long-established sound system may be the result of a change of environment - for example a new chemical works downwind of the structure under discussion.