Explain the differences among corrosion, erosion, and oxidation in turbine engines and how they are detected during overhaul.

• A. They are the same process; detected by sound tests only.
• B. They are detected only by temperature measurements.
• C. Corrosion is chemical attack; erosion is material loss from particle impact; oxidation is a high-temperature reaction forming oxide scales; detected by visual/NDT, metallurgical checks, and coating wear.
• D. They are different forms of wear that require metallurgical analysis only.

Answer: (C)

Corrosion, erosion, and oxidation are distinct degradation mechanisms in turbine engines, each with a different cause and signature. Corrosion is chemical or electrochemical attack on metal from the engine environment or fluids, leading to pits, thinning, or cracking. Erosion is actual material loss caused by high-velocity particles impacting surfaces, which wears away material especially at exposed edges and flow paths. Oxidation is a high-temperature chemical reaction with oxygen that forms oxide scales on hot components; these scales can thicken, spall, or crack and may accompany coating degradation.

Overhaul detection uses a combination of visual inspection, nondestructive testing, metallurgical analysis, and coating wear assessment. Visual checks reveal pits, thinning, discoloration, or coating loss. NDT methods such as dye penetrant, magnetic particle, eddy current, and ultrasonic thickness gauging detect surface cracks and measure remaining material or coating thickness. Metallurgical checks examine microstructure and identify oxide layers or corrosion products, while coating wear assessments indicate whether erosion or high-temperature oxidation has compromised protective coatings.

This distinction—chemical attack for corrosion, mechanical wear from particle impact for erosion, and high-temperature oxide formation for oxidation—along with the integrated inspection methods, is why this explanation accurately reflects how these processes are identified during overhaul.