This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
All electro deposits of metal are slightly porous, and so when a thin deposit of nickel on steel or iron is exposed to moisture the tiny drops penetrate these pores to the metal beneath and cause rust. A thicker deposit offers abetter protection, or better still is a coat of Copper deposited on the parts and well burnished previous to being coated with nickel.
 
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