Soil Corrosion and Caustic Corrosion Q&A for the exams

By Eng. Carlos Molina

To download 52 questions for your own study, based in SOIL and CAUSTIC corrosion in the API 571 RP, click here

Both DM ares listed in the API 571 RP under the “Uniform or Localized Loss of Thickness”


One of the most common damage mechanisms present in tanks. It is like atmospheric corrosion because it is related to the environment outside the storage tank.

Although it seems straightforward, soil Corrosion is a very complex damage mechanism. API 5751 says: “The severity of soil corrosion is determined by many factors including operating temperature, moisture and oxygen availability, soil resistivity (soil condition and characteristics), soil type (water drainage), and homogeneity (variation in soil type), cathodic protection, stray current drainage, coating type, age, and condition.” Of these, the parameter that is used with most frequency to determine soil corrosivity is soil resistivity (related to moisture content and dissolved electrolites). Acidity of the soil is also a contributing factor.


This is part of a group of damage mechanisms that are generated predominantly due to corrosive conditions on the product side. Also, it predomionantly causes localized corrosion, specially gouging, on account of which it is called caustic gouging too.

According to the API 571 RP, Caustic Corrosion is “Localized corrosion due to the concentration of caustic or alkaline salts that usually occurs under evaporative or high heat transfer conditions. However, general corrosion can also occur depending on alkali or caustic solution strength.” Presence of NaOH and KOH is the major contributing factor to caustic corrosion. And there are many uses fot these 2 in the industry. Three from my research:

  • for extracting mercaptans from liquified petroleum gas (LPG). link  
  •  to “scrub” harmful acidic gases (like SO2 and H2S) produced in the burning of coal and thus prevent their release into the atmosphere. link   
  • for removal of organic acids and sulfur compounds  link   


With the pdf here available, now there are 383 Q&A of the API 653 series. We still have a lot of work to do though. We have covered 7 of the 11 damage mechanisms in the BOK so far.

Thanks for reading.


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{ 3 comments… add one }
  • mamughal November 18, 2016, 5:24 pm

    APIExame Articles are very informative and helpful for better undertaing of the subjects

  • Sriram January 3, 2017, 4:20 pm

    Dear Sir,

    Kindly guide on filling the application form for API 653 as a self employed person.

    Awaiting your reply, thanks!

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