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Overview of Pitting Corrosion

Pitting Corrosion is a fairly localized form of corrosion which is characterized by small holes, known as pits, forming in materials, most often metal piping. While these pits may be small on the surface, they can be larger underneath, potentially being more damaging than they appear. The materials that are most vulnerable to this type of corrosion include stainless steels, nickel alloys, and aluminum alloys.

This type of corrosion can be particularly dangerous to facilities due to multiple factors. One of the reasons is because the pits formed as a result of this mechanism are so small, they are fairly difficult to detect. Even more than that, the pits can be hidden from view by thin coatings or membranes. It can also cause damage to materials very rapidly, making it particularly insidious.

Pitting corrosion tends to be caused by environmental factors. It can be caused by certain corrosive materials such as chloride, hypochlorite, or bromide. Fluorides and iodides can also be harmful. The development of pitting can be sped up by the presence of water or sulfides in the environment. These chemicals cause anodic and cathodic reactions when in contact with certain metals which results in localized galvanic corrosion.

Thankfully there are several ways in which pitting can be mitigated and prevented. The best way to prevent pitting is construct your piping out of materials that are resistant to any pitting causing chemicals in the environment it will be working in. In general, higher alloys tend to be better at resisting pitting corrosion. If that isn’t possible, cathodic or anodic protection can also be used to prevent the chemical reactions that are occurring in the environment that cause pitting from affecting the pipes. Controlling the pH, chloride concentration, and temperature of the environment is another possible way to prevent pitting.

 

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Cathodic Protection Coatings Condition Monitoring Locations (CMLs) Corrosion Control Documents (CCDs) Damage Mechanisms Flue Gas Dew Point Corrosion Green Rot Positive Material Identification (PMI)
Articles about Pitting Corrosion
  • July/August 2018 Inspectioneering Journal
    By Greg Garic at Stress Engineering Services, Inc.

    API 579-1 is a complex document covering several different types of equipment that may contain flaws or damage. Due to its complexity, this article condenses it into six things you need to know.

  • March/April 2003 Inspectioneering Journal
    By Dr. Nand K. Gupta at Omega International Technology, Inc.

    During the past two years, a new High Resolution Gamma Ray Detector Array System to detect and measure small corrosion pits in real-time in the inservice piping in process industries, has been in development. This High Resolution Gamma Ray Detector Array can be substituted in place of the standard detector array in our ThruVU (TVU) system. The first High Resolution detector array has a total width of 1.00" and consists of 76 channels with 0.013" detector pitch. On the other hand, the detector pitch is 0.130" in the standard TVU detector array. So, the new High Resolution detector array can potentially provide a 10x better spatial resolution compared to the TVU standard detector array.

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