Evolution of moisture barriers on metal jacketing
Moisture barrier protection on metal jacketing or cladding over thermal insulation is now accepted as the best way to protect the integrity of the system. Initially when Aluminium or stainless steel showed signs of corrosion on the exteriors such as pits and perforations, it was thought that the material was failing due to external factors, further investigation into this revealed that the damage was being done from within.
Over time the joints of any metal jacketing can be susceptible to water ingress through the gaps at the overlaps which are caused due to the failure of the sealants because of mechanical abuse allowing pathway for moisture to enter the system. In cold work applications, the insulation system is generally sealed with a vapour barrier and tape to prevent any moisture ingress that could lead to corrosion of the pipe, commonly known as corrosion under insulation (CUI). As a result, any water entering the system would sit between the vapour barrier and the metal and this is where the issue lies. The trapped water acts as electrolyte leading to pitting and crevice corrosion in these confined spaces.
Another issue to contend with is galvanic corrosion, this is generally caused by the reaction of dissimilar metals coming in to contact with an electrolyte (such as moisture). Stainless steel can be more at risk in higher temperatures so will also effect hot systems. As this is a well-known fact and insulation engineers will allow for this by keeping materials such as aluminium foils away from stainless steel jacketing to prevent any dissimilar metal contact. However it’s worth noting that in recent times, this has failed due to contractors using foils without barriers or the barriers becoming damaged and having bare aluminium break through.
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