PCS PCS Corrosion Science and Prevention 1 — Questions and Answers
Question 1: In electrochemical corrosion, what four components must be present simultaneously for the corrosion cell to function?
- Anode, cathode, electrolyte, and metallic path (Correct answer)
- Oxygen, water, iron, and carbon dioxide
- Primer, topcoat, substrate, and electrolyte
- UV light, moisture, salt, and metal
Correct answer: Anode, cathode, electrolyte, and metallic path
A complete corrosion cell requires an anode (where metal is lost), a cathode (where reduction occurs), an electrolyte (ionic conductor), and a metallic path connecting anode to cathode.
Question 2: Which form of corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte?
- Crevice corrosion
- Galvanic corrosion (Correct answer)
- Stress corrosion cracking
- Erosion corrosion
Correct answer: Galvanic corrosion
Galvanic corrosion occurs when two dissimilar metals form an electrochemical couple in an electrolyte, causing accelerated corrosion of the more active (anodic) metal.
Question 3: What type of corrosion occurs in low-oxygen areas such as lap joints, gaskets, and under deposits?
- Galvanic corrosion
- Uniform corrosion
- Crevice corrosion (Correct answer)
- Intergranular corrosion
Correct answer: Crevice corrosion
Crevice corrosion develops in confined spaces where oxygen is depleted, creating an oxygen concentration cell that accelerates localized attack within the crevice.
Question 4: Zinc-rich primers provide corrosion protection to steel primarily through which mechanism?
- Barrier protection by sealing the surface
- Cathodic (sacrificial) protection by acting as the anode (Correct answer)
- Passivation of the steel surface
- UV absorption to prevent photodegradation
Correct answer: Cathodic (sacrificial) protection by acting as the anode
Zinc-rich primers protect steel cathodically because zinc is more active than steel in the galvanic series, so zinc corrodes sacrificially to protect the steel substrate.
Question 5: What is the primary cause of osmotic blistering in coatings applied over steel in immersion service?
- UV degradation of the resin
- Water-soluble salts trapped under the coating film (Correct answer)
- Insufficient film thickness
- Excessive surface roughness
Correct answer: Water-soluble salts trapped under the coating film
Soluble salts (chlorides, sulfates) beneath a coating draw water through the semi-permeable film via osmosis, creating blisters filled with a corrosive electrolyte solution.
Question 6: Which test method is used in the field to detect soluble salt contamination on a prepared steel surface before coating?
- Pull-off adhesion test (ASTM D4541)
- Bresle patch method (ISO 8502-6) (Correct answer)
- Cross-hatch tape test (ASTM D3359)
- Roughness comparator (ASTM D4417)
Correct answer: Bresle patch method (ISO 8502-6)
The Bresle patch method (ISO 8502-6) extracts soluble salts from the surface using a latex cell filled with deionized water, which is then measured with a conductivity meter.
PCS PCS Corrosion Science and Prevention 1
In electrochemical corrosion, what four components must be present simultaneously for the corrosion cell to function?