Base Metal Surface Preparation

Base Metal Surface Preparation

Base Metal Surface Preparation - Magnesium alloys are usually supplied with an oil coating, an acid pickled surface or a chromate coated surface for surface protection during shipping and storage. Surfaces and edges must be cleaned just before welding to remove all paint, coatings, oxides, dirt, grease and other foreign matter.
Mechanical cleaning with stainless steel wool, power wire brush or abrasive cloth may be satisfactory for removing oxides and chemical coatings. Chemical cleaning as compared to mechanical cleaning:

(i) Gives longer protection, more uniform cleaning and lower surface resistance (when spot welding).
(ii) Is faster and gives better results.
(iii) Is done with the help of following solution:

Chromic acid	24oz*
Ferric nitrate	5.33oz
Potassium fluriode	1/16oz
Water	to make 1 gallon

This cleaning solution is kept in a tank made up of ceramic or stainless steel or lined with lead or synthetic rubber.The job is dipped in the bath, which is kept at 22 to 32°C, for about 3 minutes, then is rinsed thoroughly in hot water and dried in air.
Before chemical cleaning, oil, etc., are removed by washing with organic solvents or by vapour degreasing in stabilized trichloroethylene. After cleaning and before welding, rigid jigging is essential, as otherwise severe buckling or distortion may result from the high coefficient of expansion of magnesium alloys.

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