Cladding Protects Vessel from Sulfide Stress Cracking in Sour Service

The HVAS thermal spray method is used to coat the internal surfaces of many types of refnery equipment. Here it is being used to apply a protective internal coating on a boiler. Photo courtesy of Integrated Global Services.

When a de-ethanizer column in a refining facility was shut down for testing and inspection in 2013, cracks inside the column were discovered. The crack depths ranged from 1 to 10 mm. Most of the cracks were located at tray ring support welds, while some were found at circumferential welds and in the column shell’s base material. All possible underlying causes for the cracks were assessed based on the column’s process parameters, service conditions, previous history, and hardness testing of material from the affected areas. The process fluid contained hydrogen sulfide (H2S). Cracks found during previous inspections had been repaired by grinding. The investigating team concluded that the latest cracks were caused by sulfide stress cracking (SSC) at locations where welds were previously repaired and had high hardness levels.

The extent of the column’s failure, the root cause of the cracking, mitigation steps, and longer term remedial actions are discussed in CORROSION 2015 paper no. 5952, “Application of HVAS Cladding to Mitigate Sulfide Stress Cracking and Hydrogen Induced Cracking in a De-ethanizer Column,” by I. Hall, T. Sabti, S. Dossary, et al. According to the authors, the de-ethanizer column suffered from H2S corrosion, or sour corrosion, which occurs when H2S gas reacts with carbon steel (CS) in a wet, sour environment. During sour corrosion, hydrogen atoms formed during the corrosion process at the metal surface will diffuse through the cross section of the CS and become trapped at laminations caused by voids and nonmetallic inclusions. When atomic hydrogen is absorbed by solid metals, hydrogen-induced cracking (HIC) can result. The hydrogen atoms combine to form molecular hydrogen (H2), which creates pressure from within the metal. Trapped H2 gathers at these locations and forms blisters as pressure builds. Eventually stepwise cracking occurs when several hydrogen blisters connect in the base metal.

In the industry it’s well understood that hydrogen permeation takes place in the steel shells of vessels,” says NACE International member Iain Hall, chief technology officer with Integrated Global Services (Richmond, Virginia). “In a sour environment, the corrosion happening at the carbon steel shell interface is relatively low and doesn’t reduce material thickness quickly. The problem is with materials that are susceptible to hydrogen-induced cracking.” He explains the hydrogen atoms that would normally pass through the CS shell in very low concentrations instead will accumulate at dislocations or weaknesses in a material—particularly at or near welds—and cause hydrogen blisters or HIC. The authors also note that in the case of SSC, any regions in the CS material or its welds that have hard microstructure and are exposed to sour corrosion can experience hydrogen embrittlement (HE) due to the high concentration of diffused hydrogen atoms, and that hardness levels for CS in sour service should be below 200 Brinell hardness (HB).1 Current standards for pressure vessels in sour service call for the use of HIC-resistant steel, which is manufactured with reduced impurities and controlled morphology to prevent laminations and trap sites where hydrogen atoms could collect.

The de-ethanizer column was fabricated in 1975, before HIC-resistant plate requirements were in effect at the refinery. Additionally, Hall notes, welds that were not postweld heat treated (PWHT) were also highly susceptible to HIC. Several mitigation methods are known to prevent SSC. One is to perform a stress-relieving treatment on the metal to reduce hardness levels to acceptable levels in an annealed microstructure. A second method is to clad the CS base material and welds with a corrosion-resistant alloy (CRA). Cladding the interior of the de-ethanizer column with a thermal spray-applied CRA would form a barrier between the CS and the sour process fluid and prevent sour corrosion, which would stop the formation of hydrogen atoms and their subsequent migration into the steel. Thermal spraying is an industrial coating process that uses either an electrical (plasma or arc) or chemical (flame) heat source to melt a coating material (powder or wire) into tiny droplets and spray them onto surfaces.

To evaluate repair alternatives for the de-ethanizer column, a risk matrix was developed. Because of the presence of hydrogen blistering and the concern that stepwise cracking would occur between the blisters, stress-relieving options were not considered. Thermal spray cladding, using a HIC-resistant NiCrMoW alloy to coat the interior of the de-ethanizer column, was selected as the repair method. This type of CRA is known to provide corrosion protection in sour environments; however, the thermal spray method selected for applying the material is also key for optimum corrosion protection, Hall says. He comments that thermal spray techniques, like welding techniques, vary and the technique selected depends on the process environment and the coating’s intended use. Unlike a metallic thermal spray coating that is designed to act as a sacrificial coating and corrode preferentially to protect the substrate, Hall notes, a thermal spray coating for this application would need to completely block the fluid’s corrosive constituents from corroding the base metal, so porosity and density of the applied coating are important considerations for preventing corrosion of the substrate.

A thermal spray coating is an accumulation of splats of deposited material on the substrate. When splats are deposited, thin interlaminar oxides can form as a result of in-flight oxidation of the cladding material. These oxide bands act as stress concentration sites as well as pathways for corrosive media to permeate the coating. A critical parameter in determining the integrity of a thermal spray coating system is the splat size, and a smaller splat size forms a denser coating with little or no porosity.2 The ability of the cladding material to form a non-porous coating is a consideration as well. Hall notes that some CRAs, when used with certain thermal spray processes, will degrade and produce a porous coating with oxides.

A high-velocity arc spray (HVAS) thermal spray method and high-performance modified NiCrMoW alloy developed by Integrated Global Services specifically for sour service cladding applications were evaluated for use on the de-ethanizer column. According to Hall, this HVAS technique reduces the oxidation of the coating material during application through an atomization process that deposits fine particles to produce a highly dense coating; and the alloy was fine-tuned to resist degradation during application and form an oxide-free, nonporous coating. Electrochemical testing, in addition to standard metallographic evaluation, was conducted to quantify the suitability of the HVAS process and modified NiCrMoW alloy. Test results indicated the coating’s corrosion performance was similar to wrought Hastelloy C276 (UNS N10276) plate, and the decision was made to use the modified NiCrMoW alloy HVAS coating.

The de-ethanizer column was coated in situ on site. “This was the first time that this particular coating had been used on such a large scale for the internal protection of a vessel,” Hall comments, noting that it previously had been used on small-scale projects to protect against hydrogen blistering. Surface preparation before coating included restorative welding work comprised of grinding and repairing SSC cracks; high-pressure water blasting; and abrasive blasting to NACE No. 1/SSPC-SP 5,3 which removed corrosion products and deposits on the shell. Any geometrical features that could contribute to poor coating deposition, such sharp edges, weld spatter, and deep grooves between weld beads on the shell and tray support rings, were removed. The HVAS coating was applied at a mean coating thickness of 20 mils (500 µm).

The vessel was later taken out of service in July 2014 for subsequent testing and inspection. An inspection team combining both vendor and client technical personnel evaluated the integrity of the applied HVAS coating. No indications of SSC or corrosive coating delamination were observed, and the vessel was returned to service with continued monitoring.

Contact Iain Hall, Integrated Global Services—e-mail:

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  1. API RP 571, “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry” (Washington, DC: American Petroleum Institute, 2011).
  2. T. Shrestha, I. Hall, “Corrosion Mitigation Using Thermal Spray Coating” presented at NACE CORROSION 2014, San Antonio, TX, 2014.
  3. NACE No. 1/SSPC-SP 5, “White Metal Blast Cleaning” (Houston, TX: NACE).

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