Guide to Corrosion

There are many types of corrosion. This page describes the general mechanisms, the definition of the different corrosion processes and the surface engineering principles that can be used to reduce corrosion.

Mechanism of Corrosion

Corrosion is an electrolytic action involving an exchange of electrons and ions. It can take place between dissimilar metals or between areas of the same metal or alloy component where there are differences in electrochemical potential. These occur naturally with the effects of oxides, impurities, alloy phases and metallurgy, but any corrosive situation requires a conducting electrolyte (moisture, salt water, caustic, etc.) to establish the electrical circuit. A typical corrosion cell can be represented by:

Metal dissolves at the anode, whilst hydroxide (OH) ions form at the cathode. The reaction between the dissolved metal and the hydroxide ions then produces the characteristic corrosion products. Hence, it is the anode that corrodes in any cell and which material is anodic to another material is dictated by the galvanic series. The detail of any relationship depends to some degree on the environment and the electrolyte, but a typical galvanic series in a saline environment would be:

 
Hence, if mild steel is attached to a nickel component, it will be the steel that corrodes. If it attached to aluminium, it is the aluminium that corrodes.

Types of Corrosion

1) General Corrosion

• There is a wide area of surface attack
• The corrosion is a result of galvanic differences across the surface
• The corrosion rate is predictable
• Addressed by barrier or sacrificial coatings

2) Pitting Corrosion

• The damage is highly localised and rapid
• It is a result of localised breakdown of surface protective films (passive layers)
• It is prompted in particular by chlorides and attacks the different metallurgical phases in the surface
• It is promoted by stagnant conditions, with the effect of gravity being important to pooling of the corrosive medium
• It can occur at the base of cracks in coatings
• It is best minimised by good design and by using defect-free coatings

3) Erosion/Corrosion

• Occurs when impinging particles or medium are present
• The erosion removes the passive layers which would otherwise protect the surface, as well as removing any stable corrosion products which would otherwise have reached equilibrium
• Needs a hard, tough coating to combat it

4) Crevice corrosion

• Occurs with bolted parts and threads when submerged in electrolyte
• The crevice creates a small anode, the remainder of the sample being a large cathode, so corrosion is highly concentrated
• Best addressed through improved design to avoid crevices.

5) Cavitation

• Bubbles in an impinging liquid implode against the surface in areas of flow where the vapour pressure is suddenly reduced
• The explosive impact exceeds the yield strength of the material
• Promoted by design faults and is best combated by tough coatings, e.g. elastomers

6) Corrosion Fatigue

• Occurs when cyclic stresses and corrosion are present together
• Corrosion at crack tip (a 'crevice') reduces fatigue strength and promotes crack growth.
• Needs shot peening, a tough barrier coating and attention to design.

7) Stress Corrosion Cracking

• Occurs when a tensile stress is combined with corrosion, including situations where there is residual stress after machining or fabricating
• Corrosion at crack tip (a 'crevice') reduces tensile and shear strength and promotes crack growth
• Needs shot peening, a tough barrier coating and attention to design and finishing techniques.

8) Bi-Metallic Corrosion

• Two dissimilar metals brought together and connected electrically, with an electrolyte bridging the junction
• One will be anodic to the other and the potential will drive a significant current, at the rapid expense of the anodic part (e.g., a copper gasket fastened between two steel flanges)
• Reduce it by insulating the couple, by applying barrier coatings to both, applying a sacrificial coating to the cathodic part (Al, Cd or Zn), or by applying non-metallic films (e.g. anodising Al Alloys).
• Can also be reduced by setting up a sacrificial part remote from the components and applying a voltage ('Cathodic Protection').

 

Reducing Corrosion with Surface Coatings

Coatings are generally used in one of two ways:

i) Apply a barrier to prevent the electrolyte reaching the component surface

For instance, painting of steel structures and applying nickel or copper coatings to steel or aluminium parts both come under this heading. Protection is effective until the coating is penetrated, either via a pore, a crack or by damage or wear. Then the substrate will corrode preferentially to the coating (since it will be anodic to the coating material) and corrosion products, which are generally more voluminous than the parent metal, will lift of the coating and allow further attack.

ii) Apply a sacrificial coating to corrode preferentially

For instance, the application of zinc, cadmium or aluminium coatings to steel parts falls under this heading. Corrosion progresses steadily, but it is the coating which suffers and not the substrate, even if the coating is porous or cracked. The issue is then one of the corrosion rate and how long the thin coating can continue to protect. Typically, it is cadmium that performs best in these circumstances, producing a slow corrosion rate (particularly if chromate passivated after deposition) with only a small volume increase for the corrosion products.

The addition of zinc or aluminium to paints and other polymeric or elastomeric coatings can also provide this galvanic protection.

For specific coatings to reduce corrosion, click below

• Apticote 200 Polymer coatings
• Apticote 300 Hard Anodising and Composite Anodising for aluminium alloys
• Apticote 400 Electroless Nickel and Nickel/Polymer coatings
• Apticote 600 Silver coatings
• Apticote 800 Thermally Sprayed coatings
• Apticote 900 Cadmium coatings
• Apticote Keronite 3000 for aluminium alloys