Chrome is a very effective material for the correct application, but it can waste a lot of time and money if used for the wrong purpose.
What Chrome Handles Well
Chromium plated parts can be made from steel or cast iron parts that have experienced some wear and have lost some surface material. This can include shallow wear, decrease in surface hardness and slight undersize. If the base metal is sound and prepared correctly then shallow wear down to about 0.5mm can be repaired using chrome.
Where It Falls Short
Chrome does not bond into cracks or other surface anomalies, therefore parts with deep fatigue cracks are not suitable for chrome plating. Parts that have undergone excessive wear resulting in gross dimensional loss (over 1mm) are generally not suitable for hard chrome plating as the build-up can create internal stress in the deposit and can result in poor adhesion. The part may then require grinding back to size and the resulting thin chrome deposit may not be economically useful.
Chrome does not perform well on parts that have deep fatigue cracks. Such cracks are not repaired by chrome plating because the chrome follows the surface of the part and does not bond into the crack to arrest its growth. Furthermore, parts that have undergone excessive wear and have therefore undergone a large amount of dimensional loss (more than about 1 mm of surface material) are generally not candidates for hard chrome plating. Although such parts can be built up to size, the build-up can create high internal stresses and the part may not have a suitable surface finish for the plating. After such a part has been ground back to size, there may not be enough material left to plate effectively.
What Hard Chrome Plating Can and Cannot Fix
Chrome is a great process for certain applications but is wasted on others.
What Chrome Handles Well
Hard Chrome Plating is a suitable repair process for wear on steel and cast iron parts. Wear of up to 0.5mm, decrease in surface hardness and slight undersize can be repaired with chrome, as long as the base metal is sound and correctly prepared.
Where It Falls Short
Fatigue cracks: Deep fatigue cracks cannot be repaired by chrome as the deposit only follows the surface of the part and does not penetrate into cracks to bond with them or to stop their propagation. Large dimensional loss: For loss greater than about 1 mm the risk of internal stress in the deposit and poor adhesion is very high. In such cases regrinding back to size may leave very little deposit for functional purposes.
Base Metal Suitability
All steels – including high alloy and high temperature tool steels – can be successfully chrome plated. Other metals and materials, however, are very difficult to plate. Aluminium and Titanium in particular present great difficulties. Even with the best of preparation the adhesion is usually poor and not worth the extra cost of processing.
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Surface Finish Limits
One further point to note is that Chrome coating replicates the finish of the surface that it is deposited onto, so a well ground steel surface can be expected to receive a coating with a surface finish of Ra 0.1 to 0.4 microns, following plating and subsequent grind. However a surface with scratches, marks, etc. will be reproduced in the coating, and it is not capable of filling in scratches etc. in the same way that other coatings can. The coating does however increase the hardness of the surface of the part.
Geometry and Coverage Constraints
Internal bores and blind holes don’t plate evenly due to current distribution issues – surface thickness will vary with possible thin areas. Parts with many recessed areas or sharp internal corners are not ideal candidates for hard chrome plating.
Regulatory Pressure
The traditional hard chrome plating solution uses hexavalent chromium, a compound which is currently restricted and requires authorisation for industrial use under the REACH regulations. Many manufacturers are now looking to move to alternative surface solutions in a bid to stay ahead of future restrictions on the use of this compound.
Chrome is a good repair material for simple parts made of steel or cast iron that have suffered a small amount of surface wear, have lost a small amount of material, are slightly undersized and have suffered a slight reduction in surface hardness, provided the parent metal is sound and is correctly prepared for chrome plating.