I have conducted submersion testing in 3.5wt% of NaCl of various different metals in combination.
One such metal I used in combination was extruded aluminum 6063 T6 with cold worked zinc plated 1018 steel and another combination was extruded aluminum 6063 T6 with cold worked zinc plated 304 stainless steel.
I measured the mass of each component 7, 14 and 21 days into the testing.
The surface area of each part in question are circa 1000mm^2 each.
What I found was very strange. The aluminum actually gained mass instead of losing.
I would be extremely grateful to anyone who might be able to proffer why this mass gain has occurred instead of loss in the aluminum in the above applications.
Thanks,
Jon
Aluminum corrodes with the formation of oxide and hydroxide layers, which are not soluble in water (practically nothing goes into the solution). During the corrosion of the aluminum, the oxide layer adheres to the corroded area and does not normally fall off. For this reason, the mass of the corroded part increases.
Dear Jon Knaggs please go through the ASTM:G1–90 "standard practice for preparing, cleaning, and evaluating corrosion test specimens" to stand on the standard methods for cleaning aluminum and aluminum alloys after testing assessment of corrosion damage. This will change the results you got toward accurate determination of the mass loss. I can send a copy of the standard if you do not have one.
Best regards
Vadim Verlotski and, Mohammed Hussein j. H. Al'Atia , Thank you both for your responses.
Vadim Verlotski - I have not been able to locate any papers that back this theory. Whilst I recognize the this process is exploited during anodizing, would you argue that due to the potential difference, that I have unintentionally anodized the aluminum?
Mohammed Hussein j. H. Al'Atia - in my current laboratory, I have very limited apparatus, although I did have a read through ASTM G1 and thank you for offer, ultrasonic cleaning is something I shall consider for my next round of testing. Could you advise why removing the oxide layer in full is so important for this style of testing?
Thank you both in advance.
Thank you dear Jon Knaggs for your feedback and the question as: why removing the oxide layer in full is so important for this style of testing?
Yes it is so important for accurate and reproducible results since the metal losses determination is based on atomic losses from the specimen's surface by anodic oxidation reaction as atomic ions which will produce always weight loss. While the oxide layer is formed by the reaction of these atomic ions with the product of the cathodic reduction reactions in 3.5wt% of NaCl solution forming molecular layer of aluminum oxide and chloride . These molecules are heavier than aluminum atoms itself which in turn divert the weight loss toward weight gain therefore this layer must removed in full to have only the weight loss by anodic oxidation reaction . For that I recommend you to follow the ASTM G1 cleaning procedure.
Best regards
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