CFS Corrosion Resistance & Coatings 3 — Questions and Answers
Question 1: Mechanical galvanizing differs from hot-dip galvanizing primarily because it:
- Uses a thicker zinc coating
- Uses tumbling with zinc powder at room temperature instead of a molten bath (Correct answer)
- Produces a harder coating
- Requires post-treatment acid wash
Correct answer: Uses tumbling with zinc powder at room temperature instead of a molten bath
Mechanical galvanizing cold-welds zinc powder onto fasteners via tumbling with glass beads and does not involve a molten zinc bath, avoiding hydrogen embrittlement.
Question 2: What does ASTM F1941 govern regarding fastener coatings?
- Hot-dip galvanizing of structural bolts
- Electrodeposited coatings on threaded fasteners (Correct answer)
- Organic polymer coatings on fasteners
- Thermal spray zinc coatings
Correct answer: Electrodeposited coatings on threaded fasteners
ASTM F1941 specifies requirements for electrodeposited coatings on threaded fasteners including dimensional and performance requirements.
Question 3: Which environmental condition accelerates corrosion most significantly on bare carbon steel fasteners?
- Low humidity with no chlorides
- High humidity combined with chloride ions (Correct answer)
- Dry indoor air at room temperature
- Pure water with no dissolved salts
Correct answer: High humidity combined with chloride ions
The combination of high humidity and chloride ions is particularly aggressive, as chlorides break down passive films and water acts as the electrolyte for corrosion reactions.
Question 4: The minimum zinc coating weight for hot-dip galvanized structural bolts per ASTM A563 nut standards is closest to:
- 0.5 oz/ft²
- 1.0 oz/ft²
- 1.7 oz/ft² (Correct answer)
- 3.0 oz/ft²
Correct answer: 1.7 oz/ft²
ASTM A153 requires approximately 1.7 oz/ft² (Class C) for threaded fasteners in hot-dip galvanizing applications.
Question 5: Grade 5 fasteners electroplated with cadmium were historically popular in aerospace primarily because cadmium:
- Is harder than zinc and more wear resistant
- Provides excellent corrosion protection and is compatible with aluminum (Correct answer)
- Is cheaper than all alternative coatings
- Has a higher melting point than zinc
Correct answer: Provides excellent corrosion protection and is compatible with aluminum
Cadmium plating provides excellent corrosion resistance, low contact resistance, and is galvanically compatible with aluminum airframes, making it historically popular in aerospace.
Question 6: Which term describes the ability of a coating to heal small scratches or damage through migration of protective ions?
- Cathodic protection
- Sacrificial inhibition
- Self-healing or galvanic sacrificing (Correct answer)
- Passivation
Correct answer: Self-healing or galvanic sacrificing
Zinc coatings self-heal minor damage through galvanic sacrificing, where zinc ions from the intact coating migrate to protect the exposed base metal.
Question 7: Thread fit after hot-dip galvanizing typically requires nuts to be:
- Tapped undersized to compensate for coating buildup
- Tapped oversize to accommodate the thicker zinc coating (Correct answer)
- Left at standard dimensions since zinc doesn't affect thread fit
- Replaced with stainless steel nuts
Correct answer: Tapped oversize to accommodate the thicker zinc coating
Hot-dip galvanized bolts have significantly thicker coatings than electroplated fasteners, requiring nuts to be tapped oversize per ASTM A563 to achieve proper thread engagement.
CFS Corrosion Resistance & Coatings 3
Mechanical galvanizing differs from hot-dip galvanizing primarily because it: