Featured Story – Featured StoCorrosion under insulation at high temperatures in the chemical industry

Austenitic stainless steel and CUI Unfortunately, the idea that stainless steels are a catch-all solution to corrosion issues is all too common across many industries, especially in the chemical and petrochemical industries. The additional costs of stainless steel often lead owners to believe they have fully protected their assets. However, these alloys are not ‘stain-proof’ they are only ‘stain-less’.

When owners truly want to protect their assets, they must assess the corrosion risks and develop specific protection strategy with ongoing monitoring and inspections throughout the design life. For this reason, corrosion under insulation (CUI) is particularly interesting to discuss, as it is one of the most difficult forms of corrosion to detect and can often have significant financial and environmental impacts.

CUI is found in a range of industries across the globe, including petrochemical, chemical, and oil and gas. In the chemical fertilizer industry, specifically in ammonia plants, some insulated pipes operate between -200°C to 400°C-500°C. Insulated stainless steel pipes are used to transport the nitric acid in these plants. Indeed, many chemical plants have lots of stainless pipes and stainless steel reactors, which also need to be insulated.

Elevated temperatures and the presence of chlorides are scourges for steel subject to CUI. Chlorides are, of course, ubiquitous. In the case of CUI, chlorides will progressively concentrate at the steel-insulation interface. The absorbent insulation will pull and hold moisture containing chlorides and dissolved salts against the steel surface. Subsequent evaporation of the moisture as the steel heats up leaves behind concentrated levels of chlorides. This can cycle repeatedly from hot/dry and hot/wet oxidizing conditions to dry ambient temperature conditions creating an extremely aggressive exposure at around the steel surface. In these environments, the high chloride content is well above the threshold to resist pitting and localized corrosion of stainless steel 304 and 316, so high nickel and molybdenum alloys (duplex, Inconel, or Hastelloys) would be required.

Without using these more corrosion resistant alloys, the most cost-effective solution to preventing CUI is the use of carefully selected immersion-grade coatings to protect the steel surface at the steel-insulation interface.

This is easier said than done. Due to the cyclical exposures there is a wide range of temperatures that can induce CUI. According to NACE SP0198, the critical temperature ranges that a high heat coating must withstand to mitigate CUI are 50°C to 175°C for stainless steel.² These conditions and temperatures are fairly standard for your average epoxies and urethanes; however, industry trends have seen more and more applications in need of protection with temperature cycles up to 300°C, and even as high as 650°C.

IMM coatings

There is a countermeasure strategy that owners can adopt to minimize risk in their asset integrity programs. Protection of the commonly used types 304 and 316 austenitic stainless steel assets from CUI is accomplished with the application of judiciously selected single component and two component inorganic multipolymeric matrix (IMM) coatings to stainless steel. Each coating has a higher bond strength and resistance to cleavage of the Si – O bond (452kJ/mol) compared to the C – O bond (358kJ/mol) in organic epoxy coatings. Thus, both confer a higher thermal stability and thermo-oxidative resistance to degradation when applied over abrasive-blasted stainless steel or carbon steel assets.

The IMM coatings were able to withstand a simulated CUI environment in previous research by the authors.⁵ Moreover, the coatings had over 10 years of proven success in real world CUI service, and operated at temperatures up to 650°C. One IMM coating contained aluminum-flake pigmentation; the other contained micaceous iron oxide pigmentation.⁶

Figure 1 shows the overlapping aluminum flakes in the former IMM coating and plate-like structures of micaceous iron oxide in the latter IMM coating provide a tortuous diffusion path against the intrusion of water, oxygen and dissolved chlorides.

Both IMM coatings were touted to confer internal stress reduction, mechanical toughening, thermal shock and crack resistance and withstand cycling and continuous operation between ambient and elevated temperatures. Figure 2 shows a schematic of the molecular structure and network of the inorganic silicon based IMM coating containing aluminium-flake pigmentation.⁷

Experiments with stainless steel and IMMs

Accelerated laboratory tests were undertaken to evaluate how IMM coatings could solve the external stress corrosion cracking (SCC) dilemma for stainless steel in CUI conditions of 50°C to 175°C, and even when temperatures cycle from ambient to approximately 600°C.

Type 4 austenitic stainless steel pipe spools were used with a nominal composition of 8-12 weight percent nickel, 18-20 weight percent chromium, and 0.08 weight percent carbon maximum, with the balance iron. The pipe spools measured 60cm in length, 5cm internal diameter, and 5mm thickness. They were abrasive blast cleaned to SSPCSP10 Near White Metal with G40 steel grit to achieve a 2-3 mil jagged profile.

Two IMM coatings were spray applied to the pipe spools. The aluminum flake pigmented IMM coating was applied in a single coat at 8 mils DFT (dry film thickness). The micaceous iron oxide pigmented IMM coating was applied in two coats of 5 mils DFT per coat to 10 mils total DFT.

As shown in Figure 3, the pipes were insulated in high-water retention insulation (5cm pre-formed calcium silicate, Cal-Sil half shells), and in a modified Houston Pipe Test vertically positioned on hot plates to a CUI microenvironment of cyclically wetting aluminum foil wrapped insulation with a 1% sodium chloride solution, cyclic heating, with a thermal gradient of approximately ambient temperatures to 600°C.⁶ The temperature of the stainless steel pipe under the insulation was determined by placing thermocouples between the uncoated steel pipe and the insulation at 5cm intervals.

The appearance of the coated pipe samples after CUI cyclic testing at high temperature is shown in Figure 4. Under the test conditions, both IMM coatings exhibited no evidence of rusting (ISO 4628-3 Rating 0), blistering (ISO 4628-2), or cracking (ISO 4628-4) along the stainless steel pipes heated to 460°C. From 460°C to 600°C, however, cracking and flaking of large coating chips of both IMM coatings (5mm to 20mm in length) was seen on the pipe’s surface. Apart from the flaking, the adhesion of both IMM coatings to the austenitic steel substrate was very good with ratings of 8-10 (ASTM 6677) in the CUI range.

The microstructure of the type 304 austenitic steel pipe is predominantly austenite with some ferrite islands present. The stainless property is achieved by the tenacious surface oxide layer that is formed due to the alloying of chromium and nickel. Type 304 austenitic stainless steels may be susceptible to sensitization at temperatures from 550°C to 800°C. At this temperature range, chromium may migrate to austenite grain boundaries, resulting in a thin zone of chromium depletion adjacent to the grain boundaries. This chromium depleted zone is susceptible to intergranular stress corrosion cracking in aggressive environments, including chloride solutions.

In general, the coefficient of thermal expansion for stainless steel is 17.3 x 10-6cm/cm°F from 0°C to 600°C. For a 25cm length of pipe, the total linear expansion expected from room temperature to 600°C is 0.26cm in length. This will remain more linear from room temperature due to the austenitic matrix present.

Some possible scenarios for the coating disbondment are significant sensitization and intergranular attack of the stainless steel during the cooler temperature cycles in the presence of the chloride solution. The thermal expansion coefficient may also contribute to the coating bond degradation in the 460°C to 600°C temperature range.

IMM coatings containing either aluminum flake pigmentation or micaceous iron oxide pigmentation were equally effective under the conditions tried.

Test panels of type 304, type 316, and type 321 austenitic stainless steels were sweep blasted with non-metallic abrasive, to achieve a jagged profile of 1.5 to 2 mils. The aluminum flake pigmented IMM coating was spray applied in both single coat and two coat applications to the panels. The latter were heated in an oven to a temperature of 450°C for 4,200 hours. After cooling, they were exposed for two years at a coastal site in the northeast of England with an ISO 12944-9: 2018 CX (corrosivity category) environment.⁸ Significantly, the performance of the IMM coating was excellent in both the laboratory testing and after exposure to real world service with no blistering, cracking, or disbondment of the IMM coating, and no corrosion of the stainless steels panels.

A final analysis

In the final analysis, were the results for the IMM coatings applied to austenitic stainless steel good, bad, or ugly? The answer: the IMM coatings were excellent. And Aristotle should have last word for us all: “Excellence is never an accident. It is always the result of high intention, sincere effort, and intelligent execution; it represents the wise choice of many alternatives-choice, not chance, determines your destiny.”