As the industrial industry works to find better ways to combat corrosion under insulation (CUI), understanding how the insulation’s chemistry can affect corrosion is a critical piece of the puzzle. Recent research has confirmed that corrosion inhibitors that are integral to the insulation’s chemistry, like the XOX Corrosion Inhibitor® in Thermo-1200® calcium silicate and Sproule WR-1200® expanded perlite, can impede rust development on the pipe surface. In these two insulations, the XOX Corrosion Inhibitor has been shown to deposit a layer of leachable silicates and ions onto the pipe surface, forming a protective layer that decreases the corrosion potential. Notably, the chemical composition of this protective layer is consistent with documented standard ASTM C795 requirements for limiting stress corrosion cracking of stainless steel.
Since the XOX Corrosion Inhibitor works by leaching silicates and ions to form a protective layer on the pipe surface, it has been questioned whether the corrosion inhibitor leaches out after repeated wet/dry cycling events. This theory would suggest that it would become less effective throughout the life of the product, potentially to the point of “wearing out” entirely in extreme cases.
With this question in mind, Johns Manville designed a new study to determine whether repeated and prolonged exposure to water ingress causes the XOX Corrosion Inhibitor to diminish over time. The test was based on the ASTM C1617 test protocol, with specific variations that would look at the corrosion inhibitor longevity in Johns Manville’s water-resistant Thermo-1200 calcium silicate. 12 samples of Thermo-1200 were cut from two separate pieces of insulation (six sample sets from each piece of insulation). The insulation samples were subjected to the ASTM C1617 test protocol after being subjected to repeated wet/dry cycles. Each wet/dry cycle included 8 hours of water submersion followed by 16 hours in a 450°F oven. Both sample sets had a control sample did not undergo any wet/dry cycling. Each of the subsequent samples were exposed to 10, 20, 30, or 40 wet/dry cycles.
The wet/dry cycling served as an accelerated replicate of conditions that could be encountered in the field. In a compromised system, during process upsets, cycling operations, or scheduled downtime, the insulation can cycle through saturation and dehydration repeatedly.
The first set of six calcium silicate samples was submerged in tap water, and the second set of samples was submerged in a 1500 ppm chloride solution. The tap water was used to mimic an inland region, while the 1500 ppm chloride solution was used to represent exposure in a coastal region.
At every tenth wet/dry cycle (0, 10, 20, 30, and 40), one insulation sample was pulled from each cycling process to undergo the ASTM C1617 test protocol. After undergoing the test protocol, Johns Manville examined the surface of the metal coupons with EDS analysis to establish whether the leachable silicate and ion content varied depending on the amount of wet/dry cycling the sample experienced. If wet/dry cycling were to affect the XOX Corrosion Inhibitor, we would expect to find that the leachable silicate and ion content would diminish in the samples that underwent more wet/dry cycles.
Instead, the results showed that throughout the test, the protective surface layer content remained relatively consistent. In the control sample (the uncycled sample) in the tap water set, the total surface composition of protective silicates and ions was 32.1%. When we compare that to the sample that was cycled 40 times, we find the total surface composition was 32.2% protective silicates and ions. In the salt water sample set, the uncycled sample had a total surface composition of 39.2%, while the material that underwent 40 cycles had a total surface composition of 34%.
The variations in the surface layer composition are minor, and they are the result of variations in the base chemistry of insulation and the limitations of the analysis techniques used. The variations are not caused by the material gaining or losing its corrosion inhibiting properties over time or because of wet/dry cycling.
As system designers must decide which insulation materials are right for the requirements of their system, it is important they keep in mind that the insulation chemistry will dictate what components leach from the insulation onto the pipe surface. Different insulation chemistries will interact with the pipe surfaces in different ways. In this study, it was clearly demonstrated that the long-term efficacy of the XOX Corrosion Inhibitor does not diminish over time or as a result of repeated and extreme wet/dry cycling. Utilising these findings can be the first step in building a comprehensive corrosion mitigation strategy and prolonging the life of the pipe and insulation systems. To learn more about Johns Manville’s XOX Corrosion Inhibitor and its extensive research on CUI, visit its CUI Resource Library.
Written by Marybeth Jones, Johns Manville.
Read part one of this article here.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/02122020/understanding-the-longevity-of-corrosion-inhibitors-in-insulation-part-two/
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