Summary for galvanizing thickness Discussion
Calculating required galvanizing thickness is the core of this thread. Edwin Pang Jian Xiang asked how to estimate the zinc layer needed to protect galvanized steel for 5 years when the current density is given as 6 x 10^-3 A/m^2, noting that the problem seemed unsolvable without mass or area. In response, the answerer worked through Faraday’s law: over 5 years, that current corresponds to about 948,000 coulombs, or roughly 9.82 Faradays, enough to consume about 321 grams of zinc, which is about 45 cubic centimeters per square meter. That works out to a total coating thickness of about 45 micrometers, or 22.5 micrometers per side because galvanizing is two-sided. The answerer also questioned whether such a current density could be measured in practice.
Q. Under particular corrosive conditions it is estimated that the current density of galvanised steel sheet is 6X10^-3 A/m^2. Estimate the thickness of the galvanised layer to give rust protection for a minimum of 5 years?
This question seems unsolvable because there is no mass or area given
Can anyone help me out?
Answer: thickness> 45 micrometer
Edwin Pang Jian Xiang
STUDENT – Petaling Jaya, Selangor , Malaysia
2005
A. I have never seen a student question quite like that one, Edwin; thanks for sharing it with us!
We rarely do people’s homework for them, but this one is interesting so let me try it.
6 x 10^-3 A of current, running for 5 years (1.58 x 10^8 seconds) would be 948,000 coulombs or about 9.82 Faradays. That should consume 9.82 gram equivalent weights of zinc or 4.91 molecular weights or 321 grams or 45 cubic centimeters.
So, if each square meter of surface area must be coated with 45 cubic centimeters of zinc, it is coated with 45 cm/m^2 or 45 cm/(100×100)cm^2 or .0045 cm thickness or 45 micrometers. Note that galvanizing is invariably two-sided, so the thickness on each side is 22.5 micrometers.
Now my question is, how can we measure that current density — I don’t think there is any practical way.
Ted Mooney, P.E.
Striving to live Aloha
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