API RP 687 - Chapter 0, Annex F: Coatings

On my last post, I was celebrating having completed the review of Chapter 0, API RP 687.

It may have been a bit premature, since today I feel compelled to cover a bit more in depth Chapter 0 Annex F: Antifouling/Corrosion Resistant/Performance Improvement Coatings.

I personally find the topic today fascinating, because it is a way to add a little color to what otherwise would be plain polished steel rotors.

Let’s start with a bit of history. In the world of turbomachinery repairs, the most popular coating I recall was a product called Sermetel 5380.

I remember reading this in many requests for quotes (RFQs) and Technical Bid Specifications from turbomachinery owners asking for this coating by name.

This fancy product, the multilayer corrosion resistance coating, was born by patent in 1963 in Pennsylvania by Allen Charlotte of Teleflex Incorporated.

If you enjoy reading patents, you can read it here: https://patentimages.storage.googleapis.com/47/26/77/a8255165812e1a/US3248251.pdf

Sermatech, a division of Teleflex iterated and developed several coating solutions before arriving at the Sermetel 5380DP in 1980.

If you are keeping track of all these company names, just know that Praxair Surface Technologies acquired the Sermetel product line in 2015.

Now, Praxair is not the only company making these types of coatings. The company I work for happens to make a product engineered to have similar or, in some cases, with enhanced qualities; including chrome-free versions to make them safer to work with and environmentally friendly.

Several other aerospace products or service companies also manufacture these types of multilayer coating solutions.

Regardless of who makes them, most of these coating consist of the following:

• They consist of multiple layers, each of a different material which provides different properties.

  
  

• They provide protection through a combination of physical and chemical mechanisms.

Protection Mechanisms

Corrosion and Erosion Resistance

1.    Dense Packing: The aluminum particles in the base coat are densely packed, creating a barrier against corrosive elements.

2.    Sacrificial Protection: When made electrically conductive through burnishing, the aluminum in the coating acts as a sacrificial anode, protecting the underlying metal substrate.

3.    Chemical Passivation: The chromate in the coating provides chemical passivation, further enhancing corrosion resistance.

Surface Properties

1.    Smooth Finish: The coating provides an aerodynamic finish, which is crucial for gas path turbine components like compressor blades.

2.    Porosity Reduction: The top coat seals any remaining porosity in the base coat, further enhancing corrosion resistance.

3.    Chemical Inertness: The top coat is chemically inert, providing additional protection against various fluids and environmental factors.

What are they made from?

There are two main classifications: Organic or Inorganic.

Organic

They are derived from hydrocarbon compounds. They are primarily used for their ability to create a robust barrier against corrosive agents.

These can operate up to 400ºF (200ºC) so are used on low temperature applications.

Inorganic

These are made from chromate, phosphate, or silicate bases and are applied to components exposed to high temperatures and severe environmental conditions.

These are good for steam turbines and high temperature gas path components, since they can withstand temperatures up to 1050ºF (565ºC).

API RP 687 wisely states that the coating selection should include an evaluation of the coating’s capabilities, an understanding of the material properties of the parts being coated and an analysis of the flow path and working fluid.

The document provides two tables that serve as a quick selection guide for coating applications.

In my experience, I have mostly seen inorganic coatings applied on steam turbine disks, blades, process compressors, gas turbine compressor sections, and gas turbine disks.

The most common being the Aluminum Chromate Phosphate coating!

This blend of metallic-ceramic components creates a chemically bonded protective layer.

The two most popular inorganic coating applications I see daily are: a two layer and a three layer coating.

Two Layers

The Sermetel 5380 or the Sulzer A08 and A21 fall into this category. They can both also be called aluminum-ceramic coatings.

The base coat contains aluminide chromate-phosphate. This layer provides corrosion protection by acting like a sacrificial coating or providing cathodic protection.

Metals oxidize when they react with oxygen and moisture by exchanging electrons. The same way an apple turns brown after it has been cut.

The aluminum in the base coat has higher electrical potential than the base metal, meaning it is more generous in giving away its electrons. So when the base metal wants to start corroding, the aluminum will sacrifice its own electrons, saving the steel.

The second or top layer acts like a sealer, the same way we protect cars with sealers or films, improving the looks and surface finish.

At Sulzer, the engineers developing the coating made the top coat blue, because we like blue.

Three Layers

Sulzer has a three-layer product, the same way I am sure others may, called A24. The third layer contains Teflon (PTFE), which enhances the surface finish and reduces the probability of fouling, making it ideal for centrifugal compressors.

Teflon is that magical stuff they put on frying pans, so eggs don’t stick! This is why we say this coating is “anti” fouling.

Fouling is when the process gas contains particulate, or contaminants that with time accumulate on the rotor parts. Imagine driving your car with a lot of mud stuck in your wheels, at high speed you will feel high vibrations.

The same exact thing happens to centrifugal impellers or expander blades. They can foul, accumulating stuff and vibrating, causing rubs and damage against the shaft and seals.

At Sulzer, engineers decided to make the top coat green so it could be easily recognized.

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This is all I wanted to share this week, a bit of history and some fun facts about how these coatings work!

Here are a few pictures of coated rotors.

A centrifugal compressor rotor coated with a three-layer coating with Teflon for anti-fouling protection.

Here is a gas turbine compressor section with a corrosion resistance two-part coating.

A beautiful steam turbine, case and rotor. The rotor is coated with a two-part corrosion resistance coating.