THE STORY ON THE SURFACE: HOW CORROSION + WEAR DEBRIS ANALYSIS ARE USED IN ENGINEERING FORENSIC INVESTIGATION

Equipment and machinery failures can lead to significant costs, including business interruption and equipment replacement.

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Equipment, component, and machinery failures can lead to significant costs, including business interruption and equipment replacement. Was the manufacturing faulty? Has it been operated correctly? Has the maintenance been insufficient? Could the failure have been predicted or avoided? The answers to these kinds of questions have significant implications for equipment providers, owners/operators and insurance companies.

Surface degradation by corrosion and wear is one of the most common reasons for equipment failure. While both can be easily identified from the appearance of machine components, the challenge to the forensic engineer is to determine how and why these failures occur.

Determining Mechanisms of Corrosion

When a susceptible alloy is exposed to the right environmental circumstances, chemical degradation can occur causing metal corrosion. For example, mild steel is a common engineering material, but it will corrode when exposed to water and oxygen. Premature failure caused by corrosion in steel is often due to the breakdown of corrosion prevention measures.

Although general corrosion can occur slowly, some corrosion mechanisms create localised aggressive conditions that accelerate through-wall corrosion, causing leaks and holes in a matter of weeks or months. A through-wall metallographic section cut from the failed component can help visualise the corrosion mechanism (e.g., cracking or pitting).

Chemical analysis of the alloy can also help identify its susceptible corrosion mechanisms. A metallurgical investigation will combine results from the chemical analysis with information from the maintenance records and water analysis. Evidence from where the corrosion occurs (e.g., localised at joints or near welds or where sludge accumulates) and how widespread it is throughout a system can help confirm the mechanism and possible causes.

Investigating Wear

Wear is the other common surface degradation failure mode. It is sometimes seen in conjunction with corrosion and affected by both the properties of the metal and environmental conditions. Wear is a concern because worn parts lose thickness, and thinned areas can burst. Additionally, moving parts can loosen and vibrate or seize up from friction heating. Although moving parts are protected from wear by appropriate lubrication, wear debris can accumulate in the lubrication oil and exacerbate the problem when the oil is re-circulated.

The forensic materials engineer can try to match sludge and solid particles found in the sump oil to wear particles still embedded in a wear surface. A high magnification examination using a scanning electron microscope can help determine particle size and shape and identify the composition of the debris. The route of investigation will depend on if the wear was caused by hard contaminant particles, loose corrosion oxides or contact fretting debris from the surface.

Preventing Corrosion and Wear Failures

Wear and corrosion can occur at a low level over a long time without causing equipment breakdown. But as they develop, corrosion products and wear debris can create problems elsewhere in machinery. If these issues can be detected and acted upon at an early stage, it’s possible to prevent a major and sudden failure. Pre-emptive maintenance, routine water and oil sampling, and condition monitoring are key to preventing breakdown.

One of the best ways to protect against corrosion is by implementing preventive measures early in the design of steel equipment and its operation. Corrosion protection typically involves the use of water chemistry treatments and barrier coatings that prevent exposure to corrosive environmental conditions. Protective measures may include adding chemical inhibitors to water storage tanks, boilers, pipework, or plumbing as well as coating surfaces with paint, plastic, or concrete.

When an equipment failure occurs, the story on the surface isn’t the full story the forensic engineer needs to tell. Looking closely at surface damage and appearance can often give key evidence for building the investigation.

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