Common Problems Of Hard Chrome Plating During Hydraulic Cylinder Rod Repair
Hard chrome plating is a critical part of hydraulic cylinder repair. Cylinder rods that have been in use for a substantial length of time will invariably exhibit some degree of wear on the hard chrome surface, which means replating the chrome is best practice when it comes to returning your cylinders to peak operating effectiveness.
However, hard chrome plating is a highly specialised process involving chemistry, metallurgy, mechanical skill, deep knowledge and extensive experience. There are many things that can go wrong – from bad electrical connections to poor bath chemistry, improper pre-plate grinding or improper racking & jigging.
here are the 8 common hard chrome plating problems you should avoid when repairing your hydraulic cylinder rod;
Dull / milky deposits:
There are a number of potential causes for poor chrome appearance, but it is most commonly caused by a mismatch in current density and bath temperature. This condition could also signify the presence of chlorides and tramp metal contamination of the plating tank such as iron and copper.
Burnt deposits of chrome are usually found near the high current density areas of the cylinder rod. When the current density is too high, the chromium is deposited very quickly. This effect is magnified in the areas receiving the most current – leading to an extreme build-up of chromium deposits usually on the edges or end of the cylinder rod.
Poor or partial coverage:
Poor coverage refers to a plated piece that exhibits “missed spots”. These may be small patches of unplated metal or large sections where the plating has not occurred. The most common cause of partial coverage is bad chemistry. Electrical connectivity issues may also cause this condition – such as poor or intermittent contact between the parts and the rack and/or a rack that cannot carry sufficient current to all areas of the part during plating. This condition is exacerbated during long plating cycles. In all cases, poor or partial coverage is not acceptable for hydraulic cylinder rod repair.
Poor adhesion refers to the peeling, blistering or lifting of the chrome deposits away from the base metal. It is caused by a weak bond between the chrome and the underlying surface. This is generally caused by a poorly prepared surface. During the plating process, chrome is deposited on to the base metal of the cylinder rod, which is (usually) a previously plated part. If this surface is not properly prepared, trace metals, oils and contaminants could be present at the time of plating. Poor adhesion often occurs when chromium is plated over existing chromium, which has failed to be removed during the pre-grinding process. Current interruptions are another cause of poor adhesion.
A rough chrome job is often the result of poor base metal condition. While other finishes such as copper tend to fill in holes and scratches in the base metal resulting in a smooth and level surface, chromium tends to follow the contour of the metal. This means any imperfections will be exaggerated, not levelled off. In addition, these raised areas attract more current, which leads to more chromium deposits in this spot – further exacerbating the problem. The end result is an uneven, rough surface finish.
Pitting is a common problem which is characterised by the presence of excessive small pinholes in the chrome surface. There are many potential causes, which makes it difficult to diagnose and deal with. Sometimes the base metal is porous. Foreign material that is not removed during pre-plating preparation can also cause pitting. Insoluble material suspended in the bath at the time of plating will have the same effect.
Micro-cracking is a normal part of the plating process. As the chromium builds up, it develops fine cracks, which are then covered over by the next layer. Micro-cracking is a somewhat desirable outcome because small cracks provide channels for lubrication, which aids in reducing friction. However, excessive micro-cracking will increase surface roughness beyond acceptable limits. Variations in temperature and chromium sulphate ratio play a role in the degree to which micro-cracking takes place.
Coarse cracking of the chrome surface is known as “mud cracking” or “chicken wire cracking” due to it’s a resemblance to dried and cracked mud, or the uneven hexagonal mesh of chicken wire. Mud cracking is usually caused by improper grinding technique – including the use of the incorrect grinding wheel, incorrect grinding speed, excessive grind depth, or inadequate or incorrect grinding fluid. Much cracking severely compromises the surface finish and makes the hydraulic cylinder chrome rod less corrosion resistant.