What is hydrogen attack

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.

Prevent

This problem can be prevented by

• Using a resistant or hydrogen free material
• Avoiding sources of hydrogen such as cathodic protection, pickling processes and certain welding processes
• Removal of hydrogen in the metal by baking.

Corrosion Main

What is stress corrosion 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.

Prevention

Prevention can be achieved by:

• Reducing the overall stress level and designing out stress concentrations
• Selection of a suitable material not susceptible to the environment
• Design to minimise thermal and residual stresses
• Developing compressive stresses in the surface the material
• Use of a suitable protective coating

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What is Fretting Corrosion

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.

Prevention

Prevention is possible by:

• Designing out vibrations
• Lubrication of metal surfaces
• Increasing the load between the surfaces to stop the motion
• Surface treatments to reduce wear and increase friction coefficient.

Corrosion Main

What is 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. This can be reduced or prevented by;

• Coating the material
• Good design that reduces stress concentration
• Avoiding sudden changes of section
• Removing or isolating sources of cyclic stress

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What is corrosion caused by 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.

Prevention

Prevention can be achieved by:

• Reducing the flow rate and turbulence
• Use of replaceable or robust linings in susceptible areas
• Avoiding sudden changes of direction
• Streamlining or avoiding obstructions to the flow

Corrosion Main

What is Thermogalvanic Corrosion

Temperature changes can alter the corrosion rate of a material and a good rule of thumb is

that 10 oC 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. The best method of prevention is to design out the thermal gradient or supply a coolant to even out the difference.

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Concentration Cell Corrosion (Crevice)

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.

 

 

Prevention

The potential for crevice corrosion can be reduced by:

• Avoiding sharp corners and designing out stagnant areas
• Use of sealants
• Use welds instead of bolts or rivets
• Selection of resistant materials

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What is Microbial Corrosion

This general class covers the degradation of materials by bacteria, molds 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

• Direct interaction between the microbes and metal which sustains attack.

Prevention

Prevention can be achieved by:

• Selection of resistant materials
• Frequent cleaning
• Control of chemistry of surrounding media and removal of nutrients
• Use of biocides
• Cathodic protection.

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What is stray current corrosion

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.

 

 

Prevention

The most effective remedies involve controlling the current by:

• Insulating the structure to be protected or the source of current
• Earthing sources and/or the structure to be protected.
• Applying cathodic protection
• Using sacrificial targets.

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What is Selective Attack

This occurs in alloys such as brass when one component or phase is more susceptible to attacke than another and corrodes preferentially leaving a porous material that crumbles. It is best avoided by selection of a resistant material but other means can be effective such as:

• Coating the material
• Reducing the aggressiveness of the environment
• Use of cathodic protection

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What is Pitting Corrosion

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.

Note: Pits can be crack initiators in stressed components or those with residual stresses resulting from forming operations. This can lead to stress corrosion cracking.

 

 

Prevention

Control can be ensured by:

• Selecting a resistant material
• Ensuring a high enough flow velocity of fluids in contact with the material or frequent washing
• Control of the chemistry of fluids and use of inhibitors

• Use of a protective coating
• Maintaining the material’s own protective film.

Corrosion Main