Corrosion, oxidation and degradation commonly spoil the aesthetics, leads to component weakening, the requirement for remedial actions and, in the worse case, failure of materials in products. Often this is directly due to the aggressive environment that a product is exposed to over its operational life. In the case of plastics or paint coatings it can be embrittlement or discolouration, paint delamination or solvent swelling. Metals can be attacked by aqueous based corrosion or oxidised at elevated temperature.
In order for aqueous corrosion to occur the following have to be present:-
- Anodic area where components degradation occurs.
- Cathodic area, the driving force for the corrosion process.
- Electrolyte to transport corrosive species and various ions.
- Electron conduction path connecting anode and cathode.
The corrosion rate is dependent on many factors, some of which are listed below:-
- Relative areas of the anode and cathode.
- Electrolyte conductivity and whether the metal is immersed in water, underground or just corroding as a result of surface moisture related to the humidity.
- Availability of fresh reactants; corrosive species such as oxygen and chloride ions, protons (H+).
- Removal of corrosion products and the tendency of the oxide to spall away which allows attack to continue virtually unhindered
- Resistance in the metallic circuit
- Electrical Resistance and durability of any protective coatings or surface treatments
- Temperature and pH.
Mechanisms to minimise the impact of the environmental attack are often in place, but have failed to be effective for a range of possible reasons:
- Unusual or unexpected exposure conditions (chemical, humidity, pH, turbulence from flowing water etc, differential aeration etc.).
- Mistaken use or wrong choice of material or metal alloy for the environment.
- Galvanic cell set up with another nearby metal component electrically in contact with the corroding part, with the circuit completed by an electrolyte, which can be as little as surface adsorbed water.
- Failed protection system.
- Ineffective or insufficient biocides or inhibitors in a closed system like a heating system as microbiological / microbial attack can be very aggressive.
The protection system designed to slow down or prevent corrosion can be
- Passivation having or deliberately growing a protective oxide. Stainless steel has a protective surface oxide and aluminium and titanium are often anodised to grow thicker protective oxides.
- Choice of material / alloy to insure the material is passive or immune in the environmental conditions the component is likely to see over life.
- Avoid two phase alloys which can provide the anode and cathode locally close together and give phenomena such as dezincification of brasses where one phase can be preferentially corroded away.
- Cathodic protection by use of a sacrificial anode or impressed current to make a key component cathodic and so not corrode.
- Anodic protection using impressed current to make a material passive.
- Painting, spraying or dip coating to provide a diffusion barrier to slow down or stop the ingress of corrosive species such as oxygen or chloride ions.
- Use of conversion coatings.
- Avoid rough surfaces as points protruding into the electrolyte will preferentially corrode.
- Cracks or small pits should be avoided as, from the catchment area principle, the anode at the crack tip (because of a lower availability of oxygen) is small with respect to the cathode and attack can be relatively fast giving pits. Aluminium and stainless steels or alloys with passive filme are susceptible to pitting corrosion.
Depending on the materials involved SEM/EDX and optical microscopy together with a combination of various surface analysis techniques, Anion Chromatography and FTIR generally are used to analyse and identify the failure mechanism and then the source of the aqueous corrosion. This can lead to the laboratory making recommendations on how to best avoid or solve the problem in the future.
Stress Corrosion Cracking and Fatigue
Corrosion, with the exception of pitting, is generally a slow process. However, it can combine with mechanical loading to produce a particularly aggressive form known as stress corrosion cracking (SCC). A tensile stress or residual stress from component manufacture helps open the crack tip open to allow the attacking elements in and concentrated chemical attack. The corrosion component of SCC allows areas crack growth to continue even when the stress is lowered.
Combining fatigue with corrosion is probably the most catastrophic form of attack affecting aluminium alloys in aircraft for example. SCC is distinct in SEM/EDX and AES (Auger).
High Temperature Oxidation
High temperature oxidation of metals occurs when the temperature is sufficient to allow interdiffusion of metal and oxygen. It can be deliberate:
- Furnace produced coloured metal for controlled heat transfer characteristics (emissivity).
- Growing a certain layer thicknesses of silicon oxide insulation or silicon oxynitride as a dielectric on semiconductor wafers.
- Furnace oxidation method of cleaning contaminants off metals prior to thermal reduction.
In other cases oxidation is to be avoided or only permitted until a protective oxide is produced. Areas of interest may be:-
- Inside a jet engine.
- Automotive engine components.
- Air leaks in a reducing or other gas furnace process such as hardening or annealing.
- Welding under a protective inert gas shield.
Surface oxidation can be evaluated by a combination of surface analysis techniques and SEM/EDX. The laboratory also has facilities to mimic thermal processes under a range of reducing and oxidising gas environments.