March 2017

Columns

Editorial Comment: Corrosion prevention starts at fabrication

Preventing corrosion begins with the choice of materials and methods used to fabricate the equipment, piping, valves and instruments that will be exposed to varying process conditions and weather. At present, designers and fabricators can select from a multitude of modern materials and methods. Once in service, effective asset integrity programs will use smart condition monitoring and, where needed, a treatment regime to sustain the useful working life of all components. In the Special Focus section of this issue, we provide several articles that illustrate good practices in vessel fabrication, corrosivity analysis and corrosion prevention treatment.

Andrew, Bob, Hydrocarbon Processing Staff

Preventing corrosion begins with the choice of materials and methods used to fabricate the equipment, piping, valves and instruments that will be exposed to varying process conditions and weather. At present, designers and fabricators can select from a multitude of modern materials and methods. Once in service, effective asset integrity programs will use smart condition monitoring and, where needed, a treatment regime to sustain the useful working life of all components. In the Special Focus section of this issue, we provide several articles that illustrate good practices in vessel fabrication, corrosivity analysis and corrosion prevention treatment.

Smart fabrication

Wet H2S generated in sour feed hydroprocessing can cause blistering, hydrogen-induced cracking, stress-oriented hydrogen-induced cracking and stress corrosion cracking. Selecting the right base material (e.g., 2.25Cr-1Mo, 2.25Cr-1Mo-0.25V) helps control and minimize some problems. However, for polythionic acid corrosion or wet H2S cracking, application of a cladding protective layer is really the only good option.

As explained in our Special Focus article “Hybrid technique for electroslag strip cladding of critical process equipment,” a number of viable options to execute cladding in large process vessels exist. There are a multitude of ways to apply this corrosion-resistant cladding, including using roll-bonded or explosive-bonded clad plates, or the application of more flexible weld cladding on a C-Mn or low-alloy-steel base material.

Better monitoring

The growing varieties of discounted opportunity crudes on the market contain certain risks for the purchaser, such as high naphthenic acid or sulfur content, which come with an ongoing risk of increased corrosion. The refiner must balance the cost benefit vs. risk, and corrosion control cost, when processing such crudes.

As described in the Special Focus article “New method to measure TAN of crude oil and refinery distillation fractions,” three issues cause problems: velocity, temperature and emulsions. A thin film of iron sulfide protects the steel from naphthenic acid attack. However, high-velocity flow and turbulence can dissolve the sulfide film, leaving the metal vulnerable to attack by naphthenic acid. At operating temperatures greater than 420°C, naphthenic acids are believed to break down into shorter-chain organic acids. In the crude oil desalter, naphthenic acids can cause upsets through the formation of emulsions.

Measuring the total acid number (TAN) of an opportunity crude helps a refiner form a corrosion management strategy. The new ASTM method D8045 uses no glass membrane that can be coated by a heavy fraction, and the analyst can vary the solvent composition to aid the dissolution of heavier crudes, such as bitumen.

Effective treatment

Using high-temperature corrosion inhibitors to process high-acid crudes began 30 yr ago, driven by economics that persist today. For corrosion risk assessment and control, ample published research exists on the inherent corrosivity of combinations of naphthenic acid and sulfur in refining systems, but not on the chemistry and mechanism of corrosion inhibitors.

The Special Focus article “Unravel the mysteries of corrosion inhibitor technology for processing high-acid crudes,” offers insights into the construction of corrosion inhibitor molecules, as well as how differences in molecules impact the performance of the inhibitor, and the risk of fouling in crude units and hydroprocessing units. Increased understanding of these concepts improves corrosion inhibitor selection and performance, as part of a crude flexibility strategy to process crudes that are discounted due to naphthenic acid content. HP

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