Alternate wet and dry cycles in the life of a system can create very serious corrosion problems mostly due to the formation of partially dry corrosion products that can further the accumulation of corrosive agents by absorption of moisture and the creation of under deposit corrosion. The rapidity of the damage growth due to these cycles can be greatly accelerated by operating at higher temperatures. This is particularly the case with corrosion under insulation or CUI, a very rapid corrosion attack due to crevice corrosion. These particularly vicious elements of a corrosion situation that should be avoided during the design of a system can be put to use when creating accelerated corrosion tests.
Alternate wetting and drying cycles can be done quite simply and smartly. The ISO 11474 standard, for example, describes an accelerated outdoor corrosion test performed by intermittent spraying of a salt solution onto the exposed pieces of equipment or systems. This severe corrosion test, also called the "Volvo" outdoor SCAB test (for Simulated Corrosion Atmospheric Breakdown), is typically run for long periods, e.g. 12 months, at a 45 degree angle of exposure with twice daily salt spray applications. In addition to providing a pass/fail screen, the SCAB test provides a reliable method to rank materials, processes and variables of interest with regard to their impact on outdoor corrosion performance. Other cyclic corrosion tests performed in cabinets can provide fast feedback, without which, a longer exposure would be required.
The ISO 11474:1998 Accelerated outdoor test by intermittent spraying of a salt solution (SCAB test)