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15 of the most common forms of corrosion on-site machinists will see

Posted by Simon Pownall on Aug 20, 2014 11:14:00 AM


    

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Corrosion occurs naturally and unavoidably.

The chances are you would have seen corrosion before, and will know that the process produces a less desirable material from the original metal and can result in a loss of function of the component or system. The corrosion product we see most commonly is the rust which forms on the surface of steel.

Here's a guide, originally compiled in the Beginners Guide to Corrosion, to 15 of the most common forms of corrosion found on-site.

Uniform

Accounting for 30% of machine failures, uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate.

Although it is unsightly it predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component.

Localised

Accounting for 70% of machine failures, the consequences of localised corrosion can be a great deal more severe than uniform corrosion generally because the failure occurs without warning and after a surprisingly short period of use or exposure.

Galvanic

This can occur when two different metals are placed in contact with each other and is caused by the greater willingness of one to give up electrons than the other.

Three special features of this mechanism need to operate for corrosion to occur: the metals need to be in contact electrically, one metal needs to be significantly better at giving up electrons than the other, and an additional path for ion and electron movement is necessary.

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Pitting

Pitting corrosion occurs in materials that have a protective film such as a corrosion product or when a coating breaks down.

The exposed metal gives up electrons easily and the reaction initiates tiny pits with localised chemistry supporting rapid attack.

Selective attack

This occurs in alloys such as brass when one component or phase is more susceptible to attack than another and corrodes preferentially leaving a porous material that crumbles.

Stray current

When a direct current flows through an unintended path and the flow of electrons supports corrosion. This can occur in soils and flowing or stationary fluids.

Microbial

This general class covers the degradation of materials by bacteria, moulds and fungi or their by-products.

It can occur by a range of actions such as attack of the metal or protective coating by acid by-products, sulphur, hydrogen sulphide or ammonia

Intergranular

This is preferential attack of the grain boundaries of the crystals that form the metal. It is caused by the physical and chemical differences between the centres and edges of the grain.

Concentration cell

If two areas of a component in close proximity differ in the amount of reactive constituent available the reaction in one of the areas is speeded up.

An example of this is crevice corrosion which occurs when oxygen cannot penetrate a crevice and a differential aeration cell is set up. Corrosion occurs rapidly in the area with less oxygen.

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Thermogalvanic

Temperature changes can alter the corrosion rate of a material and a good rule of thumb is that 10oC rise doubles the corrosion rate.

If one part of component is hotter than another the difference in the corrosion rate is accentuated by the thermal gradient and local attack occurs in a zone between the maximum and minimum temperatures.

Combined action

This is corrosion accelerated by the action of fluid flow sometimes with the added pressure of abrasive particles in the stream.

The protective layers and corrosion products of the metal are continually removed exposing fresh metal to corrosion.

Fatigue

The combined action of cyclic stresses and a corrosive environment reduce the life of components below that expected by the action of fatigue alone.

Fretting

Relative motion between two surfaces in contact by a stick-slip action causing breakdown of protective films or welding of the contact areas allowing other corrosion mechanisms to operate.

Stress cracking

The combined action of a static tensile stress and corrosion which forms cracks and eventually catastrophic failure of the component.

This is specific to a metal material paired with a specific environment.

Hydrogen damage

A surprising fact is that hydrogen atoms are very small and hydrogen ions even smaller and can penetrate most metals.

Hydrogen, by various mechanisms, embrittles a metal especially in areas of high hardness causing blistering or cracking especially in the presence of tensile stresses.

Next steps

Deciding which form of corrosion your machinery or tooling has can be hard, but hopefully this guide will give you the information you require to diagnose the problem.

For more information on how to treat the problem, sign up to our blog here to get the 2nd part of this blog The on-site machinists guide to treating common corrosion direct to your inbox.

Download our on oil and gas on site machining case study pack

Photo Credits: Wermac

 

Topics: On-site machining, Oil and Gas Industry, corrosion

   

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