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FAQ/AA-200 Question 3

Q:
Prospect is trialling CHEMEON AA-200 on 380 die-cast. They are trialling on Oxalic, Sulfuric/Oxalic and Sulfuric. They have tried with 4 g/l aluminum at 10,15 and 20% acid concentrations. They want to build 2 mil hardcoat thickness, but burning is an issue with CHEMEON AA-200 and they aren't seeing good results. Any ideas, comments or insights?
 
A:
Anodic thicknesses can range from .3 mils up to 1.5 mils depending on the quality of the castings and surface finish. Castings that are time-saved or mechanically surface finished afford the best opportunity for maximum oxide growth. However, caveat's abound even with a good surface finish: silicon content; distribution and segregation of silicon; copper content (380 die-casts have a copper content of 3-4%; recall that 2024 alloys have a copper content of 3.8-4.9% and the difficulty in preventing burning); random Ra surface finish on different areas of the part, etc. 
 
In this situation the aluminum levels are borderline low: we recommend a range of 6-13 g/l. Aluminum levels below 4 g/l run the risk of burning especially during the ramp. Oftentimes 40-45º F will contribute to greater oxide growth using controlled current. Preparing the die-cast surface in the pre-cleaning stage with a nitric/HF (CHEMEON 3400-3600) will help immeasurably by removing silicon off the surface. (Silicon does not convert to an oxide as it dissolves throughout the oxide and is an impediment to a linear coating.)
 
Note: The reasons for burning are attempts to add oxide beyond the oxide thickness equilibrium and the high copper content and, as stated earlier, very low free aluminum bath levels. If this shop is running by amps and not getting the desired thickness it may be advantageous to back off the amps, limit the voltage and dwell the parts beyond the voltage set point. This will still not guarantee the desired thickness; but it may help. Again, die-castings traditionally are the most notorious alloys to achieve desired and constant oxide thicknesses.
 
In terms of the processes they are trialling with CHEMEON AA-200- in general, 
- straight oxalic with CHEMEON AA-200: CHEMEON has no history using CHEMEON AA-200 with oxalic acid 
- sulfuric with oxalic acid and CHEMEON AA-200: CHEMEON has no history using sulfuric acid, oxalic acid and CHEMEON AA-200
- straight sulfuric with CHEMEON AA-200: CHEMEON has a strong successful history processing wrought and die-cast aluminum alloys using CHEMEON AA-200 in concentrations of 6-8% for hard anodic coatings.