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I have a 2 week experiment. I put aluminum coated and non-coated steel of the same size and shape in separate beakers with NaCl solution. Both are sealed. But, I am surprised that Al coated steel gained more weight than the non-coated steel after immersion. I know that the weight gain can be attributed to the formation of corrosion products. The measurements shows that the coated steel gained more weight, which be indicates more corrosion. But I'm sure that it doesn't since the liquid solution is still clear without any sign of rust. Unlike on the non-coated steel which has formation of rust that settles at the bottom.

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  • $\begingroup$ I think it is galvanic corrosion where Al acts as sacrificial anode. So Al gets oxidised. This way you are passivating the underlying steel. In principle, the behaviour should revert. The originally Al coated sample is now protected by a mechanically strong layer of Al oxide should stop getting weight, and by waiting enough, the uncoated one should start losing weight. It is steel anyway, so its corrosion may take long. $\endgroup$ – Alchimista Jul 22 '17 at 20:53
  • $\begingroup$ In general, one experiment is not representative and you always want to do series of them to collect statistical data and calculate error margin, e.g. for the average mass deviation in your case. Also aluminium in direct contact with steel in seawater points to galvanic corrosion, the question is which steel it was and how concentrated solution was in order to estimate potentials for cathode-anode pair in solution. Most likely Al cathodically protected Fe, but I'm not sure. Staneless steel can also be a deal breaker. $\endgroup$ – andselisk Jul 22 '17 at 20:56
  • $\begingroup$ But I have 20 replicates. $\endgroup$ – Acid Jul 22 '17 at 23:14
  • $\begingroup$ The nail coated steel is heavier than the non-coated steel. But I can see that the non-coated steel is heavily corroded unlike the coated steel. Does it mean my gravimetric measurements is inconclusive? $\endgroup$ – Acid Jul 22 '17 at 23:23
  • $\begingroup$ Perhaps the non-coated steel corrosion products are either more soluble or less physically robust. So they go into solutions or fall off the uncoated nail. $\endgroup$ – matt_black Jul 23 '17 at 10:20

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