What causes blackening of sand-cast CF8 stainless steel parts after pickling?
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The furan resin process was used; after the first acid washing, defects still remained on the surface of the castings. After rework and repair, further acid washing led to several issues: 1) Once the acid washing was complete and the castings were taken out, the stainless steel surface appeared shiny, but some areas turned dull during transportation; 2) Acid washing failed to give the surface a silver-white color. Only through prolonged sandblasting (over 2 hours) and acid washing (over 4 hours) can qualified CF8 stainless steel castings be obtained barely. Is there any experienced expert who can help me analyze the possible reasons?
Distinct zebra stripes; there was also a black one that looked like a sika deer :(