Products Finishing

SEP 2018

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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Page 27 of 59

26 SEPTEMBER 2018 — extended time in the low caustic step, thus causing the surface defects to reemerge and limiting the masking effect of the acid etch process. Figure 4 presents the macro-images of samples A3, B and C4; the streak marks are located to the left side of the profiles. Sample B demonstrates the highest amount of defect masking, while the marks on Sample A3 are the most visible ones. Figure 5 illustrates the SEM analyses of Samples A3, B and C4, showing the surface morphology after the acid and low caustic etching steps are completed. The results show that Samples B and C4 exhibit similarity in the shape and distribu- tion of the pores, with smoother sides and less intermetallics. On the other hand, Sample A3 has pits with sharper edges, and the intermetallics are still detectable, caused by the shorter time in low caustic step. Combining this data with the findings from the first set of samples and Figure 4, it is safe to say that the amount of dissolved aluminum in a low caustic bath has a determining effect on the final morphology of the surface. To obtain the maximum amount of defect masking effect from the acid etch process, the aluminum concentration in the low caustic bath has to be carefully adjusted, since the lack of dissolved aluminum can cause over-etching and thus resurfacing of the defects, or the excess amount of dissolved aluminum can lead to uneven etching as a result of the elevated time in the bath. Conclusion The results of these experments indicate that decreasing the concentration of aluminum in a low caustic step can also lead to problems such as preferential etching, causing Table 4. Surface Gloss Values of the First and Second Set Samples at 60°C (A-E) SAMPLE GLOSS A 23 A2 26.2 A3 27.4 A4 29 B 27.8 C 22 C2 24.6 C3 26 C4 27.2 C5 30.4 Table 5. R Values of Samples that have Similar Surface Gloss in the First and Second Sets of Samples (A3, B, C4) Sample Ra (μm) Rq (μm) A3 1.02 1.27 B 0.936 1.18 C4 1.21 1.50 Figure 3. The second set of samples, all pretreated in acid etch bath at 40°C for four minutes and post-treated in a low caustic bath containing 60 g/l NaOH, and (a) Sample A3 with 100 g/l dissolved Al at 40°C for 12 minutes and 10 g/l dissolved Al for 1 minute at 55°C; (b) Sample B, 50 g/l dissolved Al at 40°C for two minutes at 55°C; and (c) Sample C4, 100 g/l dissolved Al for 3.5 minutes at 55°C. Figure 4. Macro-images of samples A3, B and C4. PLATING

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