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Coating Testing for Corrosion Under Insulation and High Heat Conditions Part 1

Subject: How to find the correct CUI coating? A look into which tests we can use to characterise the coating materials' likely resistance to heat.

Curious about how to select the correct coating for environments with corrosion under insulation? In a series of three posts, Hempel Oil and Gas Segment Manager, Simon Daly, will look into the types of testing, which can be carried out to ensure correct specification and selection of coatings for high heat and Corrosion Under Insulation (CUI) environments. Have a look at the first post here.

Many operators in the protective coatings business have seen the horror pictures of corrosion under insulation and appreciate that coatings can play a very important part in prevention of this problem. This has been known for many years and it makes sense that a number of those horror pictures were probably once newly coated pipes.

But what makes a coating system not provide the level of resistance expected in an insulated environment?

Of course, we can look at many of the usual culprits: Poor application, application of an incorrectly specified product, or, more rarely, a defective product itself. However, I would like to look at just one specific aspect, namely how we go about determining if a coating is suitable or not for an environment where corrosion under insulation can occur, and so avoid specifying an unsuitable material.

The role of temperature in coating specifications
CUI is widely described as occurring in the temperature range of 50 -175°C, although in the case of environments with high relative humidity – the so-called ‘sweating’ pipe syndrome – corrosion beneath insulation can occur below this temperature.

Of course, in the case of a modern paint specification this temperature range is only one of many, generally resulting in specification of a variety of different paint systems. It is also fair to say that when specifying coating systems for new construction projects, protecting against CUI is simply one of many coating uses to be considered, albeit an important one.

Then there is the issue of temperature itself. We very often categorise coatings by the temperature range that they can withstand but what do we really mean by that; minimum temperature, maximum temperature, maximum constant operating temperature, peak excursion temperature etc.? Many different terms are used and this may affect our product recommendations significantly.

An example of this is pipework coated with an Epoxy Novolac coating. Routinely operating at a “temperature” of 160°C beneath insulation, we would not expect this temperature to pose any issues for this material. However, subject it to regular steaming out or process cycles to a peak operating temperature above 200°C, and it would not be unreasonable to expect to see degradation of the coating over a period of time, ultimately manifesting itself as coating failure. In the absence of a protective coating, CUI can then start to work its destructive cycle.

Also, how often do we arrive at our temperatures from process information or engineering drawings? The reality between process temperatures and skin temperatures can be significant. This may take the pipe external temperature very clearly in to the range at which CUI occurs. However, there is another thing to consider here. Coatings that work well at high temperatures should also work well at lower temperatures, right? Wrong! Many protective coatings undergo a transformation when exposed to heat. In some cases, this is necessary, such as in the case of some silicone resins where heat is necessary to remove the organic binder and leave behind the hard heat resistant silicon matrix. Definitely they will show different performance characteristics depending on the thermal conditions they have previously been exposed to.

Relevant coating tests
So how can we better categorise coating materials for high heat and CUI type environments?

We can break down the types of testing we could consider into three main types:

1)    Those that we can use to characterise the material and its behaviour under thermal conditions.
2)    Tests that indicate the performance of the coating material when exposed to a real or simulated CUI environment.
3)    Tests that could reasonably be needed to determine a coating suitability for multiple uses, for example if we wish to use a coating for insulated and uninsulated environments what other proof of performance should we consider in addition to 2) above.

This post will cover several of the different tests under type 1 - tests we can use to characterise the coating materials' likely resistance to heat.

Coating Characterisation Tests
We can use many of the standard coating terms we regularly see on product data sheets as a guide to what the material looks and feels like. Generic type, volume solids, VOC level, density are to name but a few. But how do we measure a coatings ability to resist temperature?

ASTM D 2485 is used to determine a coating’s resistance to heat. This test exposes coated panels to a variety of different temperatures where they are left for a period of time. They are then allowed to cool off (either by air or water immersion), and inspected visually for any signs of blistering, cracking, flaking and delamination from the metal surface.

Following this, panels are exposed to a corrosive environment via accelerated corrosion testing for a limited time, or a real life atmospheric corrosion test. The purpose of this is to identify any areas where heating of the coating has caused cracking, which may have penetrated to the substrate and subsequently be a site for corrosion. In this case, micro-cracks and CUI are very similar - what you can’t see can definitely hurt you.

Microscope evaluation of fibre reinforced thick film silicone after exposure to 650C reveals no significant micro-cracking

You may also use a more detailed analysis by optical microscopy in lieu of the visual inspection. This allows more thorough inspection of micro-cracking and characterisation of crack width and crack length. Research by the Danish Technical University revealed one form of micro-cracking to be an oxidative mechanism with cracking initiating at the surface and progressing through the coating as it is exposed to longer periods of heat and oxygen exposure at the base of the crack.

Modified versions of this test carried out at low temperatures can determine a coating’s ability to withstand cryogenic conditions.

ASTM D2402 describes the use of thermogravimetric analysis (or TGA). This uses highly accurate measurements of mass loss as a small sample of coating material is heated. This can be useful to characterise the polymer backbone in a coating material and the temperature at which degradation begins. Significant mass loss may also be indicative of porosity within the final coating film.

Other methods of thermal analysis such as Differential Scanning Calorimetry (DSC) can be used to highlight key events in the thermal profile of a coating. Events such as glass transition temperatures and the degradation onset temperature can all help to paint a picture of the coating material’s behaviour about heat exposure.

Hopefully, this post will have provided an indication of some of the issues which make it important to ensure we correctly specify coating materials for CUI environments, but also how knowledge about the candidate coatings behaviour towards heat will help assist in correct material selection.

Please look out for the Part II of this series of posts where we start to look at which test protocols can be used to predict a coating material’s behaviour when placed in a CUI environment, allowing us to focus further on the correct coating selection.

Simon Daly, Hempel Oil & Gas segment manager recently elaborated on the CUI topic for a broad audience at the NACE Corrosion Under Insulation workshop in Jubail Saudi Arabia. For an overview of the events which Hempel participates in, please visit our Events Calendar.


To know more about CUI and the products and services Hempel provide, please visit Hempel at Booth 4D083 at CCFW2016 this May!

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