Corrosion Under Insulation CUI (for exam takers)

By Carlos Molina

To download 38 more questions for your own study, click here


Corrosion Under Insulation (CUI) is the corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation or fireproofing.

Probability of failure in Corrosion Under Insulation is drove at least by 10 critical factors, making CUI one of the most common types of damage mechanisms. It attacks more agressivelly pipes of 4″ diameter or less. That is why in some cases, if possible, designers will choose stainless steel over carbon steel when there is probability of CUI in piping. Besides, the older the piping or equipment and insulation system, CUI is more likely to appear.

Corrosion under insulation

Some locations where CUI is more likely to occur

CUI is a potentially harmful form of corrosion, difficult to predict with certainty and difficult to find without 100% insulation removal. So the best idea is to have a good insulation from the start. As a designer, you should ake sure your tank and piping has the best insulation design, given that maintenance of insulation is also costly. A good summary of strategies to prevent Corrosion Under Insulation can be found here

As a tank inspector, you should know that the defense against CUI is using appropriate coatings under well maintained insulation barriers. It can be present at the lower portions of tank shells, and in protrusions of the isolation barrier, as many other locations. As for the API 653 exam, I don´t remember any specific questions about CUI in my exam back then in 2012, but you should study it, because it is in the BOK.

Now you have downloaded a new pdf with 38 more questions taken from parts described in the BOK for the March 2015 API 653 exam. With this, we reached 240 questions in the API 653 questions series.


More pages on the API 653 questions series

1. Beginning the path: API 653 Questions

2. General and definitions. #2 in the API 653 questions series.

3. In the core of API 653. Path # 3.

4. Damage mechanisms for API 653 inspection. Path #4


  1. In-depth explanation of standards
  2. Unsusbcribe at any time

{ 2 comments… add one }
  • zahir haddad April 25, 2016, 3:10 pm

    hello gentleman’s ,
    Mr Molina, I’d like to thank you so much for sharing those very useful information, also thank you to all this ppl that leave valuable comments on here.
    –I’m wondering if anyone on here has any tips or updated 653 exam questions ”2016” either that you remember from taking the rest or just snicked elsewhere?
    –I was also looking for real case study for calculation shell thickness?
    I would like to thank you in advance for reading and sharing 🙂 my email is
    Houston , TX

  • srinu moturi June 4, 2017, 2:39 pm

    Dear sir,
    I’m a Chemical Engineer and want to step into Oil and Gas industry. For that I have applied for API 571 exam. But, without having experience in Oil and Gas industry, its somewhat difficult to understand the concepts and crack the exam.
    So, if possible please send API 571 Question bank as you are maintaining for API 653.

    Many thanks in advance.

Leave a Comment