CFS Corrosion Resistance & Coatings 2 — Questions and Answers
Question 1: Which coating process involves immersing fasteners in molten zinc at approximately 840°F (449°C)?
- Electroplating
- Hot-dip galvanizing (Correct answer)
- Mechanical galvanizing
- Sherardizing
Correct answer: Hot-dip galvanizing
Hot-dip galvanizing immerses fasteners in molten zinc at around 840°F to form a thick, alloyed zinc-iron coating.
Question 2: Galvanic corrosion between two dissimilar metals is most severe when the metals are:
- Close together on the galvanic series
- Far apart on the galvanic series (Correct answer)
- Both active metals
- Both noble metals
Correct answer: Far apart on the galvanic series
The greater the separation between two metals on the galvanic series, the higher the driving voltage and the more severe the galvanic corrosion.
Question 3: Which type of corrosion occurs in the crevice between a fastener head and the mating surface due to oxygen depletion?
- Pitting corrosion
- Crevice corrosion (Correct answer)
- Intergranular corrosion
- Erosion corrosion
Correct answer: Crevice corrosion
Crevice corrosion initiates in confined spaces where oxygen is depleted, creating an aggressive local environment.
Question 4: Dacromet coating is primarily used to avoid which problem associated with electroplated fasteners?
- Poor adhesion
- Hydrogen embrittlement (Correct answer)
- Color inconsistency
- High cost
Correct answer: Hydrogen embrittlement
Dacromet is a water-based coating applied at low temperatures that eliminates the hydrogen embrittlement risk inherent in acid pickling and electroplating processes.
Question 5: What is the purpose of a chromate conversion coating applied after zinc electroplating?
- To increase coating thickness
- To passivate the zinc and extend corrosion resistance (Correct answer)
- To add lubrication
- To improve electrical conductivity
Correct answer: To passivate the zinc and extend corrosion resistance
Chromate conversion coatings passivate the zinc surface, sealing microscopic pores and significantly extending corrosion resistance.
Question 6: ASTM B117 salt spray testing is used primarily to:
- Measure coating thickness
- Compare relative corrosion resistance of coatings (Correct answer)
- Determine fastener hardness after coating
- Evaluate hydrogen embrittlement susceptibility
Correct answer: Compare relative corrosion resistance of coatings
The ASTM B117 salt spray (fog) test provides a standardized accelerated environment to compare the relative corrosion resistance of coated fasteners.
Question 7: Which stainless steel series is most susceptible to intergranular corrosion after welding if not properly treated?
- 200 series
- 300 series austenitic (Correct answer)
- 400 series ferritic
- 600 series precipitation hardening
Correct answer: 300 series austenitic
300 series austenitic stainless steels sensitize in the 800–1500°F range during welding, causing chromium carbide precipitation and intergranular corrosion susceptibility.
CFS Corrosion Resistance & Coatings 2
Which coating process involves immersing fasteners in molten zinc at approximately 840°F (449°C)?