Letter 27050

Shock rod macrocracks in chrome plating [Brazil] 

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We are a shock absorber manufacturer and we have a chrome platting process to protect the shock rod. We had a problem of microcracks on some rods. I already know that micro cracks are supposed to exist in chrome plating and we have an specification of 100 to 400 micro cracks per centimeter which we can see in a microscope after etching the sample. However in this case we can see them with a magnification of 100x without etching. The are causing the shock to leak oil. Some operators have described them as "Chicken wire". We are also considering them macrocracks instead of micro. I would like to know the potential causes and the remedy.

Best regards,

Carlos A. Fedosi
Quality Engineer 1st tier automotive - Lavras, MG, Brazil

Carlos.

How has your shock rod been handled before plating by machining, shootpeening and heatreatment. Micro cracks can be found with a magnification 100 x. I belive that you can see is macro cracks and they will be around 1-20 cm2 and from a Sargent's chrome solution and from a SRHS chrome solution abouth 300-800 cm2.

regards

Anders Sundman     surface finishing engineer Sweden

HI,

PISTON ROD FOR SHOCK ABSORBER SHOULD PASS THE 120 H SALT SPRY AND 1000 HV HARDNESS. THE MORE THE MICROCRACK DENSITY THE MORE HOURS SALT SPRAY TEST. IF U WANT TO KNOW THE CRACKS TYPE SEND YOUR HARD CR PLATED PISTON WITH 30 MIC THICKNESS TO SALT SPRAY TEST . IF IT PASSES 100 H OR MORE BE SURE THE CRACKS ARE MICRO TYPE OTHER VISE MACRO CRACK EXISTED. ANOTHER WAY TO SEE MACROCRACKS IS PLATING CUPER ON CROSS SECTION OF ROD , CRACKS OF MACRO TYPE EXTEND TO SUBSTRATE.

BUFFING AFTER HARD CR INCREASE CRACK DENSITY,FOR HARD STEEL THESE MICRO CHANGE TO MACRO UNLESS POST PLATING BAKING ADDMITED.

Hadi Khosravi
- Tehran, Iran

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