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

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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 intergranular corrosion

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.

 

Prevention

It can be avoided by:

• Selection of stabilised materials

• Control of heat treatments and processing to avoid susceptible temperature range

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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.

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What is Galvanic Corrosion

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
• An additional path for ion and electron movement is necessary.

 

Metals have to be significantly different in terms of their nobility in the specific environment that they are exposed to. This can be looked up in a table.

Ranking in sea water (top of table is most noble)

Noble

Platinum

Gold

Titanium

Stainless Steel 316 (passive state)

Brasses

Tin

Lead

Stainless steel 316 (active state

Carbon steel

Aluminium alloys

Zinc

Magnesium

 

Prevention

Prevention of this problem is based on ensuring that one or more of the three features do not

exist.

• Break the electrical contact using plastic insulators or coatings between the metals.
• Select metals close together in the galvanic series.
• Prevent ion movement by coating the junction with an impermeable material, or ensure environment is dry and liquids cannot be trapped.

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What is Uniform Corrosion

It is responsible for 30% of failures due to corrosion.
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 its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component. Uniform corrosion can be slowed or stopped by using the five basic facts;

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