API RP 687: Chapter 0 Annex G - Restoration Methods

Annex G describes several restoration methods; in particular, methods that allow us to add material to a part.

API RP 687 does not really explain what all these methods consist of; it just provides some guidelines and warnings, so I will expand a little of what they are.

The document lists 5 methods used in industry. In no particular order, I list them below:

• Plating

• Metalizing

• Thermal Spray Coating

• Plasma Spray

• Welding

Plating

Plating is a surface finishing process in which a thin layer of metal is deposited onto a substrate. This process is used for various purposes, improving corrosion resistance, reducing friction, and increasing wear resistance.

The most common types of plating are:

Electroplating: This involves using an electric current to reduce dissolved metal cations so that they form a coherent metal coating on an electrode. Common metals used in electroplating include gold, silver, nickel, and chromium.

Electroless Plating: Unlike electroplating, this method does not use an electric current. Instead, it relies on a chemical reaction to deposit the metal layer. This process is often used for plating non-conductive surfaces.

Anodizing: Although not technically plating, application is sort of confused into this category. Anodizing is a related process where the surface of a metal, typically aluminum, is converted into a durable, corrosion-resistant oxide layer.

In my personal experience, plating is the least favorite method for “restoration”, meaning we are adding material to a structure to compensate for wear or recover from rub damages.

I see plating used more as a surface enhancement, but not as a structural repair.

Metalizing, Thermal Spray and Plasma Spray

The next three items on the list - Metalizing, Thermal Spray and Plasma Spray - are really all types of Thermal Sprays.

This is what happens when you get a lot of people in a room to try to agree to something and they run out of time.

A better categorization could look like this:

And below, a comparison of the temperatures and flame/particle speeds achieved by each:

Here you can observe how the temperature and speed (kinetic energy) vary, and these both have an important effect on the resulting coating.

A process like Wire Flame Spray operates at a lower velocity and temperature than the other two. In consequence, the density and adhesion of the coating will be less compared to the other two.

HVOF propels particles at above Mach 2! This produces highly compacted and dense coatings with a low number of voids or inclusions.

Plasma, on the other hand, melts particles to above two times the temperature of the surface of the sun! (The surface of the sun is at 10,000 ˚F). This means that this process can melt a variety of metals and ceramics used to enhance the high temperature resistance of materials.

Welding

The last process mentioned is welding. And by now we have covered this several times, and going over it again would be like beating the proverbial dead horse.

Before specifying or accepting welding or any of the other methods, a repair provider should be able to explain the process, the pros and cons of each, and demonstrate how they have validated the use of each for the proposed repair.

There is always a lot more information in API RP 687 than what I have the energy and ability to share on my posts.

If you are an engineer in this industry, I recommend you acquire a copy.

You can read it and, in parallel, read my posts for more insights!

You can get the RP 687 here:

https://www.apiwebstore.org/standards/687