Behaviour of Fe-Al Galvanic Cell in Various Aqueous Solutions

Hot-dip Aluminum Coating on Steel: Rept. 7

Abstract

The e. m. f. of mild steel/Al or Al alloy cells in various aqueous solutions at room temperature was investigated for one week. Al and Al alloy electrodes coupled with mild steel were pure Al (99.99%), commercial Al (98%) and commercial Al containing 5% Si, 0.2% Be or both.
In city and distilled water, 98%Al+0.2/Be alloy usually protects steel as sacrificial anode, but other Al and Al alloys reverse the potential within 100hr. In 1M or M/10 sodium chloride solution, every Al or Al alloy provides sacrificial protection to steel, except in the case of 99.99%Al or 98%Al which shows potential reversal after longer time immersion, In dilute sulphuric acid solutions, Al is nobler (cathodic) than steel at the instance of immersion but immediately steel becomes cathodic. Then the potential difference gradually decreases and finally Al becomes cathodic. Only 99.99%Al does not reverss the potential except in very dilute M/6000 solution. In the aluminum sulphate solution, the potential is almost always reversed.
As described above, Al or Al alloy does not always protect the steel, accordingly, the coating should prerequisitely be free from pinholes or cracks. But the addition of Be to Al improves the resistance to corrosion.