Protection of steel exposed to high temperature areas, both atmospheric and insulated,
provides some of the most demanding situations faced by coatings. Systems applied in these areas are required to give not only resistance to high operating temperatures (which are often cyclic) but also to provide good corrosion protection. Despite this, the Protective Coatings industry still lacks a recognized test protocol for the evaluation of high temperature coating systems prior to field use. This paper describes the problems faced in the development of a thick film high temperature anti-corrosive coating for use up to 400ºC, and outlines the properties and performance results of these new generation high temperature coatings in comparison to traditional coating systems. In addition, the development of internal test methods used to try and mimic on site situations within the laboratory are also discussed.
Selection of an appropriate and correct high performance system for high temperature areas, such as piping and stacks, is often considered to be something of a ?dark art?, with premature breakdown and failure a common occurrence. Typically coatings used in these areas are too intolerant for general site application and require tightly controlled surface preparation and application procedures. In addition to the various external stresses faced by ambient exposed coatings, those operating at high temperatures have to endure further effects, including:
? Thermal cycling ? rapid expansion and contraction due to process requirements.
? Ponding/water resistance ? as a result of plant shutdown/ambient sections of a process cycle.
? Thermal shock ? occurrence of rainfall onto coating at high temperature
Cyclic temperature service in particular presents exceptional challenges to coatings. Cycling of coated steel through severe temperature gradients often causes the steel and coating to contract at different rates, huge stresses within the coating film result and often an insufficient flexibility can result in coating failure. Rapid cooling and reheating may produce condensation, which through the process of evaporation creates salt residues which are deposited on the vessel surface, and hence an extremely aggressive environment can be created. This type of thermal cycling is typical of what can
occur at shutdown or start up of processing equipment. Furthermore, if the coating in question is then subsequently insulated the potential for failure is increased. Ingress of water to produce wet insulation can cause severe problems(1) and bring into question the coating?s ability to withstand hot, humid and often cyclic conditions.
