Thermal Cycling in CUI: Why Wet–Dry Salt Concentration Can Be More Critical Than Temperature Alone

Why Temperature Bands Are Useful But Incomplete

Operating temperature is an important CUI screening factor. It affects whether water remains liquid, evaporation and condensation behaviour, coating performance, corrosion rate, chloride concentration, stainless steel SCC susceptibility, and insulation system degradation. However, temperature alone does not identify water ingress points, insulation damage, chloride contamination, wet-dry cycling, intermittent service, local geometry and retention points, or supports, terminations, and penetrations where damage actually concentrates.

What Thermal Cycling Does Under Insulation

When a system heats up, water trapped under insulation partially or fully evaporates. Any dissolved salts, chlorides, or corrosive species in the water are concentrated and deposited on the metal surface or insulation interior. When the system cools, fresh moisture may re-enter or condense, dissolving the concentrated salts and creating an aggressive electrolyte. Over many cycles, this process can create locally aggressive corrosion conditions that significantly exceed what steady-state temperature modelling would predict.

Implications for Carbon Steel

For carbon steel, repeated wet-dry cycling with salt concentration can produce rapid localized pitting, preferential corrosion at contact points, and accelerated corrosion rates at terminations, supports, and penetrations. The rate of damage may exceed that expected from the nominal operating temperature alone.

Implications for Austenitic Stainless Steel

For austenitic stainless steels, the concentration of chlorides through wet-dry cycling creates conditions favourable for chloride stress corrosion cracking (SCC). The risk depends on chloride concentration, temperature, residual or applied tensile stress, and material susceptibility. Even systems with bulk operating temperatures outside the most susceptible range may experience local temperature excursions during cycling that fall within the chloride SCC window.

CUI Inspection Must Be Location-Driven

Effective CUI inspection for cycling systems must be location-driven rather than temperature-driven. Priority inspection locations should include insulation terminations where water enters and salts concentrate; pipe supports and shoes where wetting and drying cycles occur; low points where water is retained longest; areas with known insulation damage or weatherproofing failures; and dead legs where flow and temperature may be more variable.

Integrating Thermal Cycling into CUI Risk Ranking

CUI risk ranking for cyclic systems should include operating temperature range (not just nominal temperature), start-up/shutdown frequency, insulation and jacketing condition, known moisture ingress history, chloride exposure environment (coastal, marine, industrial atmospheric), material of construction and SCC susceptibility, and consequence of failure at each location.