plating, anodizing, & finishing Q&As since 1989
Aluminium pickup/crack after anodized process
The base material is aluminium alloy 6063, pipe shape with OD 50 mm and wall thickness of 0.75 mm. Surface finishing is turned finished, and need to go for anodizing process with following parameter setting;
~soaking with hot temperature for 5mins;
~etching for 30 seconds;
~nitride for 1 min;
~anodizing for 30mins, temperature at 24 °C;
Problem occurred after anodizing, more than 50% of the parts will have pickup/crack problem on the surface, and it only able to been seen more clearly after dying process. May I know the followings;
~what are the contributors to the pickup/crack on the anodized surface?
~is my parameter setting OK for anodizing?
~what is the impact if anodizing temperature goes lower or higher?
~how to eliminate the pickup/crack problem?
CH'NG SENG KEONG
I do not think it is crack if just because of white dots after dying the anodized part. Have you ever checked it with micrometer whether it is current insulated?
High temperature and current density of Type II tend to cause white dots and powdery because too fast built-up coating and influence of Mg.
What is your current density in Anodising ?
at 24 °C, 30 mins, current density above 1.3 - 1.5 A/dm2 will result thickness above 25µm, crackling in the hot sealing
in a general way, try not to exceed 22°C for regular sulphuric acid anodising.
Anodizing pores form exactly perpendicular to an absolutely flat surface. However, the pores form necessarily spread on the curved surface of your round part. As the pores get thicker and thicker you can see the cracks in the spread. This is illustrated by sketches in the technical book, Aluminum How-To. However, we usually do not see a problem until we get into hard coat thicknesses, so this may not be your whole problem.
If the "turned finish" was done with dull tools, then you may have work hardened the surface, which when anodized will pull loose, but usually not like straight line "cracks".
Your cycle is OK.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
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