Corrosion Under Insulation: The Challenge and Need for Insulation (Part1 of 4)

When it comes to corrosion under insulation (CUI), I tend to call it a minefield, and for more than just one reason. It’s not only a hidden phenomenon, but it's also one that can really come back to bite companies who don’t invest in a good CUI mitigation strategy, because sooner or later, those companies are going to get their names in the papers due to unwanted accidents, production loss and environmental damages.

But when the proverbial horse has bolted, a lot of asset-owners want to close the stable doors by implementing rigorous measures. The problem is that they often only focus on a short-term solution, such as dismantling the entire thermal insulation, in some cases welding where wall thickness loss is critical, recoating equipment or piping and then reinsulating - in most cases with the same insulation system as before.

With CUI, it's best to think about long-term solutions and minimizing future failures. Of course, this takes an effort at all phases of the life cycle and starts with senior management allocating enough budget.

In this article, however, I'll only be focusing on what I think is the first step in the life cycle: Engineering. 

Engineering a Corrosion Under Insulation System

From engineering point of view, we need to look at the whole system and make choices about:

1. Metallurgy for equipment/piping
2. Surface protection, such as coating or metalization
3. Insulation material
4. Cladding or jacketing 
5.  

In this article I will discuss discuss items three and four in depth, and share recent insights and practices.

What Is CUI?

In recent years, I’ve seen a variety of people dealing with CUI who have little or no background in the basic physical principles of corrosion. So, I'll start with a short summary. CUI is a collective noun for various types of corrosion mechanisms, but it’s always caused by the presence of (rain)water containing chlorides and/or sulphates. For carbon steel, CUI only occurs in combination with a failed or damaged coating system, and can manifest as pitting corrosion or uniform wall thickness loss. For austenitic stainless steel, the most common form of corrosion is external chloride stress corrosion cracking (CL-ESCC).

Water, which eventually can accumulate onto the substrate and could form an electrolyte, can originate from various sources.

• Rainfall or heavy mist
• Drift from cooling towers
• Deluge systems
• Process leakage or spillage
• Condensation within the insulation systems

CUI can be expected on piping and equipment operating between -4°C (25°F) and 175°C (347°F), but also systems operating outside this range. (For example, cyclic temperatures or dead legs can increase susceptibility.) As said before, CUI is a hidden failure. It can occur locally or can affect a larger area.

Although corrosion rates for carbon steel are, in general, lower than chloride external stress corrosion cracking (CL-ESCC) rates, for carbon steel, especially near salty, seaside environments, corrosion rates of up to 20 mils (0.5 mm) per year have been reported.

Source: Johan Sentjens, January 30, 2015

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