Five Types of Corrosion Every Refinery Engineer Should Know

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Corrosion is material degradation caused by a chemical or physical reaction with its environment. In most cases it involves the transfer of electrons, an electrochemical process that water accelerates because water conducts electricity so well. Corrosion rates are typically measured in mils per year, where one mil equals one-thousandth of an inch.

Refineries deal with corrosion constantly. High temperatures, hydrogen-rich streams, acidic gases, and chloride-containing water all create conditions where equipment degrades faster than it would in a benign environment. Knowing the main types, and what causes each one, is foundational knowledge for anyone working in or around a refinery.

1. Intergranular Corrosion

Metals are made up of grains, tiny crystalline structures, with boundaries between them. Intergranular corrosion attacks selectively at those grain boundaries, which can be more chemically reactive than the grain interiors. One common trigger is welding: when a metal is heated and then cools, the grain boundaries can become depleted of protective elements, leaving them vulnerable. This version is called weld decay. It is not always visible to the naked eye, which makes inspection and proper post-weld treatment critical.

2. Pitting

Pitting is among the easiest forms of corrosion to spot during a vessel inspection, when it is advanced enough, it is hard to miss. It is highly localized, creating small but deep perforations in the metal surface rather than uniform thinning. Chlorides, fluorides, and thiosulfates in the presence of water are common culprits, particularly in stainless steel and aluminum alloys. A pit that looks small on the surface can penetrate far deeper than its diameter suggests, which is why pitting is considered especially dangerous despite its small footprint.

3. Hydrogen Embrittlement

Hydrogen embrittlement shows up in two distinct refinery contexts. The first is high-temperature, high-pressure hydrogen service, hydrotreating units being the clearest example. In these environments, hydrogen can diffuse into carbon steel and react with carbon to form methane, removing carbon from the metal in a process called decarburization. The result is loss of ductility and potential cracking or blistering.

The second context involves hydrogen sulfide (H2S). Atomic hydrogen, essentially a lone proton, can penetrate carbon steel and, when it meets another hydrogen atom, form molecular hydrogen that becomes trapped in the metal. Enough of this trapped hydrogen leads to blistering, cracking, and embrittlement. Amine regenerator overheads, where H2S concentrations are high, are a typical area of concern.

4. Stress Corrosion Cracking

Stress corrosion cracking (SCC) results from the combination of sustained tensile stress in a metal and a corrosive environment, neither factor alone would cause the same damage. Welding is a common source of residual tensile stress: as the metal heats and cools, it can be left in a state of internal tension. Post-weld heat treatment (PWHT) is the standard remedy, relieving those residual stresses before the equipment goes into service.

SCC is highly chemistry-specific. Chlorides, for instance, can initiate cracking in certain stainless steels and aluminum alloys, which is why material selection and process chemistry need to be considered together. Magnetic particle testing and dye penetrant testing are the most common field methods for detecting SCC.

5. Fretting Corrosion

Fretting corrosion is different from the others: it does not involve electron transfer. Instead, it results from repeated rubbing or sliding contact between surfaces, which wears away protective oxide layers and causes localized damage at the points of contact (called asperities). It can be reduced by lubricating contact surfaces, minimizing vibration, or sealing the interface.

A practical example: flutter valves in distillation trays. During a tower inspection, trays can show distinct grooves worn into the metal where the valve has vibrated repeatedly over a run. The valves no longer seat properly and the trays need replacing. It can look like erosion at first glance, but the mechanism is fretting, a useful distinction to make when diagnosing the root cause and deciding how to prevent it next time.

The Bottom Line

Corrosion is not one problem, it is five different problems that look different, happen for different reasons, and require different responses. Knowing which type you are dealing with shapes every decision that follows: which inspection method to use, which repair approach is appropriate, and which process or material change might prevent a recurrence. Water and chlorides show up repeatedly across the list, if there is one practical takeaway, it is to pay close attention whenever either one is present.