Question

a) A thick steel plate of total area 840 cm² (both sides) is exposed to air near the ocean. After 1 year it was found to have lost 502 g in weight due to corrosion. What is the rate corrosion, or corrosion penetration rate CPR, for the material in this environment? Density of steel = 7.9 g/cm3                               [8 marks]

b) What type of corrosion is this? [2 marks]

c) Provide two strategies (there are more than two of course, but just choose two) for dealing with this type of corrosion [2 marks]

d Briefly summarise the pros and cons of each strategy as they might apply to a children’s playground at a seaside beach front [8 marks].

Answer 1

Ans a) We know,

CPR = K W / A t

where, K = Constant = 87.6

W = Weight lost in mg = 502 gm or 502000 mg

   = Density = 7.9 gm/

A = Area = 840

t = time = 1 year or 24 x 365 = 8760 hr

Putting values,

=> CPR = 87.6 x 502000 / (7.9 x 840 x 8760)

=> CPR = 0.756 mm/year

Ans b) The corrosion may be pitting corrosion which is caused by effects of chloride ions in seawater. Pitting corrosion occurs when their is difference in ion concentration through a surface defect.

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Ans c) Two strategies for dealing with pitting corrosion are :

1) By polishing the surface of steel or by galvanization

2) By heat treatment

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Ans d) Pros of polishing surface :

1) Cost is less

2) Effective in long run if maintained

Cons of polishing surface :

1) Coating surface can be harmful for health if consumed

2) Coating surface can be removed if scratched

.

Pros of heat treatment :

1) Very Durable

2) Can result in corrosion reduction by about 50%

Cons of heat treatment :

1) Manufacturing Cost is high

2) Can result in loss of strength