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-----How to expose weld cracks before anodizing?
Quickstart:
In the anodizing of aluminum the components are immersed into a tank of acidic solution and connected to the anodic (+) positive pole of a power supply. The current separates some of the water in the solution into positively charged hydrogen and negatively charged oxygen. The oxygen is attracted to the positively charged components and converts the skin of the aluminum to aluminum oxides.
Readers new to anodizing of aluminum may wish to view our "Intro to Aluminum Anodizing".
Anodizing includes steps which will expose weld cracks, but what is sought here is a simple step to expose those cracks before the wasted effort of anodizing parts, only to find them unacceptable due to weld cracks. Read on ...
Q. We are welding 1/8" 5052 aluminum for the Navy and have had a problem with hot cracks being exposed after type II anodizing. We built some test panels (2 each from 5052 and 5086). No cracks were found after a good visual inspection prior to anodizing, but several were found in each test piece after anodizing. Is the anodizing exposing/making worse existing cracks? Is there a good compound (dilute NaOH?) that we can use to etch the welds prior to anodizing to find and repair the cracks?
Thanks for your help.
Mechanical Engineer - Louisville, Kentucky, USA
January 18, 2011
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A. I would be more interested in finding out what the problem with your welder/equipment is. You should not have cracks in Mil Spec welding.
Spray-on dye penetrate and a cheap black lite will do what you asked for.
- Navarre, Florida
A. Aluminium can be prone to smearing which could cover cracks or porosity in welds if they are being dressed back prior to anodising.
For this reason many aerospace companies requires you to etch the surface prior to inspection (particularly fluorescent dye penetrant). You are right about NaOH. Swab etching is commonly 20% solution for around 30 seconds.
Aerospace - South Wales, United Kingdom
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