Finding clarity amid ethanol-related stress corrosion cracking issues

Sept. 5, 2017
Find strategies for Mitigating Ethanol and Methanol Stress Corrosion Cracking.

ETHANOL stress corrosion cracking (e-SCC) is a vexing problem. But Russell Kane is sure of one thing: API Bulletin 939e can help.

“One of the things we struggled with is that there are certain operations and plants that have had a record of multiple incidents, and other plants that have never seen it—it’s never happened,” said Kane, president of iCorrosion LLC. “Having walked the plants and talked to people, I have found that rarely is there an obvious difference between the ones where there are failures and ones where there hasn’t been. And just because something doesn’t fall within this guideline doesn’t mean it’s unsafe or unreliable.

“Older equipment may not conform exactly to API 939e, but this does not imply that such equipment is being operated in an unsafe or unreliable manner. It is recognized that facilities may vary and may need to be modified depending on specific operating conditions and inspection and maintenance experience. Each user company is ultimately responsible for its own safe and reliable plant operations. You do what you have to do to maintain it.”

His presentation, “API Bulletin 939e: Identification, Mitigation & Prevention of Ethanol SCC,” was part of a panel that discussed the topic, “Strategies for Mitigating Ethanol and Methanol Stress Corrosion Cracking.” The panel discussion took place during the International Liquid Terminals Association’s 37th Annual International Operating Conference in Houston, Texas.

The rest of the panel: Lewis “Chip” Locke, senior project manager at Kinder Morgan; John Beavers, corporate vice-president and senior principal engineer in DNV GL’s Pipeline Services Department; Chuck Corr, biofuels technical service manager for ADM Inc; John Farrell, segment engineer technical authority for storage tanks and pipeline for BP Corp; and Daniel Leslie, project engineer at Marathon Petroleum Co.

Kane said he was a consultant for API Bulletin 939e, which is designed to help those in the industry to find, fix, and eliminate e-SCC.

He said this is what we know about ethanol stress corrosion cracking:

•  Ethanol processing methods, ethanol source, aeration, and water content (maybe chlorides) can influence SCC susceptibility.

•  In many cases, high-stress intensity is required to initiate e-SCC, and it’s in areas where there was no post-weld heat treatment (PWHT) of welds and stress concentration.

•  Lab and field information both suggest that the severity of SCC is not constant over time. Failures can occur anywhere from six months to 10 years after a plant or equipment are placed in service.

“So there are still a lot of variables out there,” he said. “I recommend API 939e. This bulletin is based on ‘lessons learned’ from API surveys and research, supplemented by industry experience.”

He said 31 cases of e-SCC were reported in the API survey efforts conducted from 2003 through 2013.

Failures most commonly have come in facility piping/fittings (24%), followed by tank floor plate seam welds (22%), tank floor/side wall fillet welds, tank sidewall first course butt weld, and tank floating roof seam welds (all at 9%), tank roof springs, facilities piping/supports, and ancillary handling equipment (all at 6%), tank nozzle, shop-built tank in E85, and pipeline (all at 3%).

Guidelines for new construction and fabrication:

•  Minimize the use of lap seam welds.

•  Minimize cold working and plastic deformation.

•  Use of PWHT, mainly piping welds.

•  Use of ethanol-resistant immersion coatings for tanks.

•  Tank foundations and pipe supports.

He said inspection for e-SCC is complicated because cracks are tight and cannot be easily seen.

“Cases of SCC have been observed in less than 12 months,” he said. “So how do you establish a minimum inspection interval? API 939e encourages prioritizing inspection based on severity of service, location, and prior cracking experience. It also encourages, where possible, ethanol sampling and analysis, destructive metal sampling, and examination for SCC confirmation.”

He said he highly recommends assessment and repair of SCC-damaged equipment:

•  Assessment of fitness-for-service and RBI, using methods like API 579 and API 581, are applicable.

•  There are possibilities for temporary patches that can be converted into permanent repairs.

•  Repairs can be made by grinding, flame or arc gouging/cutting, and welding.

“We really focus on a lot of effort on getting people to do PWHT of piping and use ethanol-resistant immersion coatings for tanks,” he said. “We all have steel, we all have ethanol, we all have oxygen. The key factor is stress. It requires high-stress intensity to get these cracks to start growing. You can reduce that by PWHT or putting a barrier over the surface, like a good ethanol coating.”


e-SCC Boot Camp

Locke said SCC occurs when material under high-tensile strength is subject to a corrosive environment.

“SCC usually occurs in certain specific metal-environment-stress combinations,” he said. “Tanks inherently have two of the factors: carbon steel and tensile stress. Tanks in ethanol or methanol service introduce the third factor: dissolved oxygen.

“The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. The problem itself can be quite complex. The situation with buried pipelines is a good example.

“Cold deformation and forming, welding, heat treatment, machining, and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The buildup of corrosion products in confined spaces can also generate significant stresses and should not be overlooked.”

He said coating is critical. The Protective Coatings Standard includes three systems for internal tank lining:

•  The tank is blasted to SSPC 10 and coated with an epoxy novolac primer with OAP (optically active pigment), then an epoxy novolac up to 24 mils thick.

•  A glass reinforced polymer using fiberglass mats to reinforce whatever resin is required for the product.

•  A glass flake system that uses the same epoxy novolac primer with OAP and then a high-solids epoxy novolac amine, glass-flake-filled coating.


Alcohol SCC: Research Summary

He said the major technical issues with ethanol are leaking of storage tanks and piping due to SCC; fuel quality; corrosion in dispensing systems (if wet); and elastomer swelling.

There is no evidence that typical line pipe or plate steels are resistant to e-SCC. Weld microstructures have similar e-SCC susceptibility to base metal. Weld metal may be somewhat more resistant.

The primary role of welding in e-SCC is to introduce residual tensile stresses. Most failures occur near welds—not in the weld microstructure. In high-pressure transmission pipelines, hoop stress from internal pressure may be sufficient to cause e-SCC.

In a factorial study involving the environment, an oxygen/steel couple was the most significant factor. Chloride, methanol, and acetic acid may be important at intermediate oxygen concentrations.

In terms of the effect of blend ratio and oxygen, dissolved oxygen is the most important factor. E-85 is susceptible and E-50 appears to be the worst.

Mitigation of e-SCC:

•  Reduce tensile residual stresses.

•  Use internal coatings for tanks. (But they will only be effective when used with grit blasting.)

•  Use inhibitors and oxygen scavengers.


Stress Corrosion Cracking in a Denatured Fuel Ethanol Environment: Producer Perspective

Corr said ethanol products are distributed through truck, rail, barge, vessel, and pipeline, both domestically and internationally.

Based on producer experience, he said:

•  There are many ethanol products, both undenatured and denatured.

•  There is a wide range of water content, typically 5% to hundreds of ppm.

•  Nearly all ethanol is stored in carbon steel tanks.

•  Not aware of SCC in any product other than denatured fuel ethanol. It’s limited to the distribution system.

•  There are no reports of e-SCC in tanks or piping at producer facilities or in transport containers.

He said e-SCC is dependent on conditions: material/metal, stress, and liquid.

He added that ethanol is not the cause of the stress corrosion cracking. Ethanol is not corrosive. It’s a solvent with specific properties: polarity, conductivity, and solubility.


e-SCC in storage tanks

He had recommendations for ethanol tank and piping practices.

For storage tanks:

•  Tank bottoms, shells, and undersides of floating roofs should be coated with epoxy coatings that are resistant to ethanol.

•  Store in an internal floating roof tank. No EFTs. Welded steel floating roofs are preferred. Bolted or welded aluminum floating roofs are acceptable. Mechanical shoe primary seal with stainless steel shoes, hangers, and hardware.

For piping:

•  All new piping components shall be subject to post weld heat treatment (PWHT).

•  Welding of pipe supports to the parent pipe should be avoided—the preference being to use clamp-type supports (U-bolts).

•  Pipe stresses should be minimized. Do not use maximum pipe support spacing. Reduce spacing to reduce stresses over supports.

Internal inspection is necessary for a tank that has been in ethanol service, regardless of duration and must include wet fluorescent magnetic particle (WFMT) of the following areas:

•  Uncoated bottom lap welds.

•  Internal corner weld (remove any existing coating, regardless of condition, to perform this inspection).

•  All uncoated internal shell welds beneath the first horizontal shell weld, including vertical butt welds and nozzle welds.

•  Uncoated carbon steel floating roof welds exposed to ethanol—if the roof is not seal-welded on the underside, the top side lap welds shall be WFMT inspected, including seams inside pontoons. WFMT requires surfaces to be cleaned to a near-white metal finish. The area extending six inches into the base metal on either side of the weld shall be prepared and WFMT-inspected.


SCC in Piping and Construction Standards

He said the following standards apply to systems with ethanol concentrations of 15% or greater.

Pipe standards:

•  Piping must meet A106 Grade B.

•  All welds are to undergo post weld heat treatment during fabrication.

•  Valve assembles are to be unibody.

•  Receipt manifold and ethanol offload line must not be connected.

Tank standards:

•  If shop-fabricated (less than 14 feet tall), the entire tank is to undergo post weld heat treatment.

Ethanol tank bottoms and walls are to be coated to at least three feet from the floor, and must be coated at least six inches above nozzles and/or manway (whichever is greater). The required coating is FastClad 105ER or Carboline Phenoline 187UHS.

•  Shell welds are to be coated above the three-foot line.

•  High-deck floating roofs in ethanol service are to be replaced.  ♦