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The thermal insulation materials used for stainless steel pipes and vessels contain chlorides. If such insulation materials are exposed to moisture, chlorides may be released into a moisture layer on the pipe or vessel surface and pitting/stress corrosion cracking may result.
While pitting and stress corrosion induced by moist thermal insulation is an important factor, the selection of pipe or vessel materials will depend on the nature of the fluid carried within the pipe or the process occurring within the vessel.
Although insulation materials contain chlorides, the leachable level of chloride in may be as low as 10 ppm. Despite the low levels of leachable chloride and the addition of corrosion inhibitors (e.g. sodium silicate) to some formulations, pitting and stress corrosion have been observed in insulated stainless steel pipes and vessels.
Regardless of the initial chloride levels within the insulation material, the presence of moisture can concentrate chloride in localised areas and lead to the corrosion at the stainless surface. The chloride level within the insulation layer may also be enhanced from external sources (e.g. rain, wash or process water) or by evaporation. In fact, rain, wash or process waters can contain as much as 10 to 100 times the level chloride that can be leached from thermal insulation materials. Such increases in chloride level can occur during storage prior to application of the thermal insulation as well as in service.
A combination of moisture, low levels of oxygen, concentration of chloride and temperature beneath the thermal insulation can lead to pitting corrosion. Stress corrosion cracking is not likely to be a factor at temperatures below 50 °C, but could be a significant feature in the temperature range 50 °C to 150 °C.
A corrosion barrier is not required for stainless steel pipes and vessels, which are not being exposed to moisture (e.g. in internal, warm and dry conditions). If, however, the thermal insulation is likely to become moist, it is recommended that, prior to application of the thermal insulation, a corrosion barrier is applied to the pipe or vessel.
Two types of corrosion barrier are used in conjunction with insulation manufactured with a waterproof finish: a suitable paint (e.g. a high temperature silicone type) or 0.06 mm thick aluminium foil.
In practice, the use of aluminium foil is favoured because of ease of application. Such corrosion barriers should be applied in accordance with BS 5970 – Code of practice for thermal insulation of pipework and equipment.