Corrosion is the deterioration of a material through its reaction with the environment. Conducting a root cause analysis to identify the exact cause of a failure of a material due to corrosion, and to determine appropriate remedies, requires a detailed and thorough analysis.
Failure analysis experts can be involved at any point of a product’s life cycle including the design, manufacturing, service, and ultimately, when a part of the product fails due to corrosion. The following article gives a brief outline of the corrosion failure analysis process.
The types of corrosion damage that are encountered in service often include general surface corrosion, galvanic corrosion, and localised corrosion. However, environment-related corrosion problems, that can be quite dangerous to a structure or product, include stress corrosion cracking, intergranular corrosion, selective leaching corrosion, and erosion corrosion.
The eight types of corrosion (Fontana) that commonly occur on structures and products made of different engineering materials are as follows:
- Uniform corrosion attack – this is the most common form of corrosion encountered.
- Galvanic corrosion – dissimilar metal corrosion
- Crevice corrosion – an insidious form of localised corrosion
- Pitting corrosion –a rapid form of localised corrosion
- Intergranular corrosion due to poor alloy metallurgy
- Selective leaching corrosion – de-alloying
- Erosion corrosion due to flowing fluids
- Stress corrosion cracking (SCC)
However, there are many less common failure mechanisms encountered as well, including high temperature corrosion; microbiologically influenced corrosion (MIC), hydrogen embrittlement, and corrosion fatigue. This latter group of corrosion failure mechanisms often require very specialised knowledge to assess and solve them.
11 key steps in a corrosion failure analysis
The steps that are often involved in a corrosion failure analysis include:
- Information gathering, often at the site of a corrosion failure – an important stage of the failure investigation.
- Preliminary visual examination, microscopic examination, and documentation of all results.
- Non-destructive testing (NDT) of samples may be required.
- Characterisation of the material properties through study of the mechanical, chemical and thermal behaviour of the failed material.
- Selection of appropriate samples for subsequent examination and analysis – a key part of any corrosion failure investigation.
- Macroscopic examination of fracture surfaces, secondary cracking, and surface condition of samples.
- The samples may also require Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRF) studies.
- Selection, preparation, and microscopic examination of metallurgical microsections of a failed metal or alloy.
- Identification of the corrosion failure mechanism. Sometimes several corrosion mechanisms may be occurring simultaneously.
- Data and results review, formulation of conclusions and reporting.
- The report should include recommendations on how to avoid a similar corrosion failure occurring in the future.
The corrosion failure analysis outlined briefly above can be very time consuming to carry out, as it may require input from a number of specialists and experts to complete. The failure analysis will also likely be expensive, with the cost depending upon the purpose of the failure investigation. However, a corrosion failure analysis carried out professionally can become the basis for a successful product liability or insurance claim. A failure analysis report may also become part of the chain of expert evidence presented by lawyers in a civil or criminal court case.
This short article by Les Boulton was first published in the ACA New Zealand Branch October Newsletter, and has used information from “How to conduct a corrosion-related failure analysis” by Steven Bradley, from Corrosionpedia