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MOSAIC CRACK PATTERNS IN CHROME PLATE

We run a small hard chrome plating facility. Recently did a flourescent dye penetrant examination of our chrome plating on a SS shaft (HRC 55). Under UV light and without magnification, there was a mosaic like crack pattern at the ends of the shaft, each mosaic about a 1mm square. There was no such crack patterns in the centre of the shaft. We use a conventional plating bath running at 40 A/dm2. The plating thickness was about 200 microns. The bath temperature about 55 to 60 °C. Prior to plating, we use TCE for cleaning. Post plating we do grinding.

Tan Chong Yin
- Singapore

Is there a possibility of higher current density at end? Can you try current cathers at end?

Payal Mag
- Charlotte

May I ask what are cathers? Are they also known as robbers which are used to draw in current?

TESSENSOHN ALBERT
- Singapore

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Can there be a problem with grinding, since edges tend to be plated thicker? Though hard chrome is relatively heat resistant, can the following grinding parameters cause structural changes to the chrome?

We use conventional grinding stones. Grit 54 and hardness K. Stone speed at 800 RPM. Stone feed about 20 to 30 microns. Material of alu-oxide for non-heat sensitive materials.

Tan Chong Yin
- Singapore

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Hi Tan Chong Yin,

It seems to me when you explain your problem. At you have a problem when you grinding the shaft. Try to use a a grinding stone with hardness L and not remowe so much every time. The cut should be 2-6 my. You don't mention if you shoot peened the shaft before hardchromium plating. All parts with 55 HRC must be shootpeened before. Because the shaft have a lots of tensile stress and also the chromium deposit. With shoot peening you will increase pressure stress.

Regards,

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden