Corrosion Protection
Corrosion Protection
This domain tests how coatings prevent corrosion and how systems are specified. Expect questions on the corrosion cell, surface preparation, salt contamination and sacrificial versus barrier protection.
Mechanism and protection
- Corrosion is an electrochemical cell with an anode, a cathode and an electrolyte. Rust forms when steel, oxygen and water meet.
- Barrier coatings exclude water and oxygen; inhibitive primers release passivating ions; sacrificial coatings such as zinc corrode preferentially to protect steel.
- Surface preparation is critical. ISO 8501-1 Sa 2½ is a common blast standard for protective coatings.
- Soluble salts on the surface draw water through the film and cause blistering and underfilm corrosion. Test with a Bresle kit and wash if needed.
- Film thickness, stripe coats on edges and welds, and correct cure all affect performance.
- Cathodic protection is used with coatings on buried or immersed steel.
Worked example
A steel tank is to be coated. The blast standard is Sa 2½ and the salt reading is above the specified limit. Applying the coating without washing will trap hygroscopic salts, which pull water through the film and cause blistering. The correct action is to wash and re-test until the reading is within limit, then coat.
Traps
- Coating over soluble salts because the surface looks clean.
- Relying on a barrier coat without stripe coats on sharp edges.
- Measuring DFT on a rusty or pitted surface and getting a false reading.
- Using a zinc-rich primer as a stand-alone cosmetic finish.