TECHNICAL DATA SHEET DURO HARD CHROME PLATING PROCESS

Transcription

1 TECHNICAL DATA SHEET Duro hard chrome plating process is a high-speed bath which employs a special mixed soluble catalyst and has been developed to meet the modern hard chrome plating requirements. Its outstanding features are, high rate of deposition and hardness of the deposited chrome layer. Due to the desirable chemical and physical properties, like high hardness and wear resistance, low co-efficient of friction and less tendency to tarnish, hard chromium deposits have found many industrial applications. The thickness of the chromium deposit to be applied depends on the expected requirements, especially taking into consideration wear and corrosion resistance. The process has several significant advantages over conventional chrome system some, of which are as under : 1. Current efficiencies are 50% and more higher, resulting in saving of energy, labour and maintenance. 2. Higher current density range. 3. Harder chrome deposits. 4. Low intital make-up concentration. 5. Easy to operate and maintain. 6. Improved corrosion resistance achieved by depositing chromium thicker than 30 microns having a large number of cracks. 7. Excellent covering and throwing power. 8. High tolerance to metallic contamination. 9. Can be operated with air agitation, thereby maintaining uniform temperature. 10. Lower co-efficient of friction and higher wear resistance of the chromium plate as a result of controlled information of cracks. The chromium deposit obtained by this process is usually cracked and offers higher corrosion resistance. Due to the inherent cracked structure of the hard chrome deposit, a small percentage of oil is retained on the surface which is of special advantage for cylinder liners and crank shafts. The oil retained on the surface, cuts down friction where during the starting period of the engine at which point the cylinder bores are normally in dry condition. So by virtue of the above phenomenon together with the good hardness of the deposits, the life of the engine parts in particular can be greatly increased. 1

2 BATH MAKE-UP Optimum Range Duro hard chrome salt g/l OPERATING CONDITIONS Density ºBe Temperature ºC Cathode current density A/dm 2 Anode current density A/dm 2 Voltage volts Agitation Cathode/Air For getting good quality hard chromium deposits, the current density and temperature should both be maintained at optimum conditions. Increase in current density should be in proportion with increase in temperature. Failure to maintain temperature will result in burnt and rough deposit at high current density areas. Similarly if the plating is carried out at a lower current density and at higher temperature, the deposit becomes milky, thus resulting in reduced deposit hardness. The base metal surface has to be smooth and uniform and surface irregularities are to be removed by mechanical polishing and burnishing operations. The parts to be hard chromium plated should be pre-cleaned in suitabale hot alkaline soak cleaner and rinsed well. A good adhesion of hard chromium deposit is obtained by anodic etching in simple chromic/sulphuric bath for 1-2 minutes and this is normally carried out in a separate tank. After plating, the parts are immersed in drag-out rinse followed by another water rinse and hot water rinse. In case of hardened steel components, the parts are annealed at 200º C for 2 hours to eliminate the hydrogen embrittlement. SOLUTION PREPARATION The Durohard chrome solution preparation is very simple. However, certain precautions must be observed in the initial make-up to obtain good results. The salts should be completely dissolved before starting the production trials. It is important that the tank linings are cleaned well. The recommended procedure is given as under : 1. Fill the clean plating tank with clean de-ionized water to about two-thirds of the operating level and heat the water to 50ºC. 2. Add required quantities of Duro hard chrome salts with stirring. The salts should be added in small increments and stirring should be continued until all the salts are completely dissolved. 3. When the salt is completely dissolved. Place the anodes in the tank electrolyse the bath for 3-4 hours at operating temperature, at amps/dm 2, until a brown chocolate film is formed on the anode. The bath is ready for plating. 2

3 EQUIPMENT Small M.S. plating tanks can be lined with soft special grade PVC Material. Bigger capacity tanks are usually of special grade PVC lined or tin/lead alloy (containing 7% tin) lined mild steel tank. In case of tin/lead alloy lined tank, loose PVC sheets are normally kept on all the sides of the tank to prevent the undesirable stray electrical currents. The plating tanks should be equipped with teflon/quartz or lead/tinlined M.S. heaters or coils. Thermostat control should be provided to maintain the temperature within the specified range. Water jacketed tanks are preferred for bigger capacity tanks since this serves a dual purpose of heating and cooling. Cooling is required to control the rise in temperature of the solution since heat is evolved due to higher operating current and voltage. For direct heating and cooling, lead / tin alloy or titanium coils are normally used. Titanium is expensive and hence teflon or similar fluoro carbon polymer coils are suitable materials for heating and cooling for chrome plating solution. ANODES Anodes of 7% tin lead alloy preferably of round shape are recommended, although flat or corrugated are satisfactory. The round anopdes are effective over their whole area whereas the other types are less effective as compared to the former. Many of the hard chromium jobs require conforming anodes. This is to maintain a fixed anode to cathode position and to improve the thickness distribution. For complicated and intricate jobs such as moulding dies, auxiliary anodes are employed. During idling period the anodes become passive and this can be cleaned mechanically by wire brush or by immersing in a alkaline cleaner. When the solution is to remain idle for more than a few days, the anodes may be removed from the solution and kept outside. On resumption of plating, anodes should be electrolysed to form protective lead peroxide film on anodes. POWER SUPPLY A D.C. power source rated 8-16 volts, depending on the current density and distance between cathode and anode is suitable in Duro hard chrome solution. The rectifier used should be of 3 phase with less than 5% ripple. The current carrying capacity of all electrical connections, bus bars and plating jigs must be designed to carry the requisite amount of current continuously. SOLUTION MAINTENANCE The solution concentration can be maintained on the basis of hydrometer reading and this can be maintained by regular additions of Duro hard chrome salts. We do not recommended the use of commercial chromic acid for maintaining the solution as this will change the ratio of chromic acid and special catalysts. 3

4 To maintain consistent results the solution should be periodically analysed for chromic acid and sulphate. The sulphate should be maintained between gm/l The active catalyst plays an important role in maintaining better efficiency, good hardness and this should be kept in the range of g/ l. The analytical method for determining the active catalyst is available on request from Growel. In case of increase in sulphate, good quality of barium hydroxide or barium carbonate is to be added at the rate of g/l to reduce the sulphage content by 1.0 g/l. To make-up for the losses during working the solution from the drag-out tank should be added and clean D.M. water (chloride free) should be used while dissolving the required replenishment chemicals. CONVERSION The conventional and other type of baths can be easily converted into Growel Duro Hard chrome bath system by adjusting the sulphate to optimum level and adding required amount of conversion additive. However, we recommend that one litre sample should be sent to Growel Laboratory for analysis and plating tests. Before drawing the sample, the solution should be stirred well and representative sample should be given for analtysis. TABLE I DENSITY OF DURO HARD CHROME SOLUTIONS Concentration of chrome salt g/l Density in degree Baume at 25 ºC TABLE II PLATING SPEED (AVERAGE THICKNESS) (Time to deposit 1.0 micron) Current density Duro Hard A/dm 2 Chromium Conventional

5 AMOUNT OF BARIUM CARBONATE REQUIRED TO REDUCE SULPHATE g/l SO 4 TANK SIZE (LITRES) to be removed Grams of Barium Carbonate required CAUTION : IT IS NECESSARY TO PREVENT CONTACT WITH EYES, SKIN AND CLOTHING WHILE HANDLING THE CHROME SALT. CARE SHOULD BE TAKEN TO AVOID BREATHING DUST FROM THE PRODUCT OR DUST FROM SOLUTIONS CONTAINING CHROMIC ACID. OPERATORS SHOULD BE PROVIDED WITH PROTECTIVE CLOTHING-APRONS, RUBBER GLOVES AND CHEMICAL GOGGLES. IN THE EVENT OF THE EYE CONTACT, WASH EYES WITH A COPIOUS QUANTITY OF WATER. IN THE CASE OF SKIN CONTACT, FLUSH SKIN WITH WATER FOR 15 MINUTES AS CHROME SALT IS AN OZIDIZING MATERIAL AND CONTACT WITH COMBUSTIBLE ORGANIC MATERIAL MAY CAUSE FIRE. FOR SAFETY DETAILS PLEASE REFER OUR SAFETY DATA SHEET NO. 0C-4. Issued on Supersedes all earlier 5