Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
Consulting Report--Hard Chrome Electroplating Durability
1. Hard chrome plated onto a smooth surface free of cracks pits and other surface interruptions plated with a minimum of 0.0007”, preferable 0.001” to 0.005”and not exposed to chlorides, acids (even mild acids) strong alkali, and without having been hit with a hard material (i.e. rocks, etc.) would be expected to have an indefinite life without corrosion. (Many years). The plating process must include the correct pre-plating preparation of alkaline cleaning, soak cleaners (no current) and/or reverse current cleaners (anodic). Direct current (parts are cathode) should not be used for high strength steels. Then a reverse polarity current treatment in a chrome solution, preferably not the plating chrome solution. The chrome plating solution using the right concentration, current density temperature and time. And the chrome plating solution is free from impurities that would lower the corrosion protection of the deposited hard chrome deposit. And is operated at the correct temperature and current density to provide the best protection properties of the chrome deposit.
2. The incoming QC test should include visual inspection for cracks, unplated areas where chrome should cover, and include measurement of the thickness of chrome deposit. Thickness tests should be made for both OD and ID chrome deposits using a nondestructive measuring tool. Use corrosion tests that will test porosity and the corrosion protection of the plated deposited chrome. It is my opinion (and experts in general) is that the common “salt spray” (ASTM B117) test process is not reliable. * See below: “Salt Spray tests.” I recommend The “Flowing Gas Test.” once or CASS Test. I understand that the call out by your customer is likely to be salt spray. Do the tests if you must, but for a more reliable test use the flowing gas test of Battelle Memorial Institute. This test is more expensive, but more reliable in detecting pores and other defects that would shorten the life of the hard chrome protective coating. There are other variations of salt spray tests that are acceptable, such as the Copper acetic acid salt spray (CASS). Both will detect porosity.
3. The surface of the items to be chrome plated, must be smooth, free from cracks roughness, pits, sharp edges tool marks and reactive metal working fluids, quenches that react with the surface. Reactant machining or grinding fluids are difficult to impossible to remove by chemical cleaning noted above. Etching will remove them but alter the finish to the extent that corrosion protection may be comprised. Use non-reactive metal working fluids. Know you’re plating company.
4. Ask for references. Visit the facility, looking for general cleanliness, a good control laboratory, and QC procedures. Ask how impurities in the chrome plating solutions are controlled, chloride removal, Trivalent chrome and iron removal, porous pot” used? Anodic treatment? Other methods?
There are a few additional means to assure corrosion protection using hard chromium. I mentioned in one e-mail that nickel used as an under coating prior to chrome plating contributes greatly to the protection. Electroless nickel with what is called high Phosphorus has the very best corrosion protection, However it is not suggested for your application due to the stress and potential bending or high vibration because electroless nickel deposit are somewhat brittle and may crack, losing its protection. Hard chrome is also brittle, and has worked for you before, so electroless nickel is not out of the realm of possibility. A very good compromise would be electroplated nickel with little or no brighteners or levelers in the solution. The deposit would somewhat ductile and would enhance the corrosion protection. Both undercoating’s would add additional cost to the plating electroless nickel being the highest cost.
The metal working fluids that you use presently are suitable for plating. Beware of silicones, they are difficult to clean in preparation for plating.
The stress relieve: 1200 is OK. Quench and temper, oil quench and wash and temper seems OK. Sand blast must use clean sand for best result. You know that of course, but monitor it if you suspect any problems there. The wash should be a good quality cleaner without silicates if possible. If silicate cleaners are used, the first rinse must be slightly alkaline, i.e. pH 8 or higher, to prevent silicates from precipitating on the surface of the items to be plated. Once on it is difficult to remove chemically.
Specifications: I suggest AMS 2406, chrome thickness specification 0.001” to .0015” Or the equivalent MIL specification. (Mil Spec. QQ-C-320). However, The ASM Handbook volume 5, Surface Finishing suggests 0.002” hard chromium for corrosion resistance applications. My belief is that if the surface is smooth, and no metal splinters or pits, that The ASM 2406 would work for your purposes. If you feel that there could be items to be plated that are not smooth, call out the thicker deposit, i.e. 0.002 inches. Note: that hard chrome plating is not sensitive to moderately rough surfaces provided no metal splinters or pits.
With the sufficient thickness of chrome plating that is agreed upon and no metal splinters or visible pits in the fabricated item, the test will not show failure, and failures in the field will stop, except for damage caused by your customer, or in shipping and handling. The life expectancy would by in many years.
Hard chrome plating solutions plate chromium deposits at the rate of approximately 1 mil (0.001”) per hour. Some Platers charge for the time in the plating solution in addition to preparation and handling.
Quality Control Testing
I recommend the use of “CASS “Copper (Acetic Acid Salt spray testing)
The main purpose of CASS testing (or any test for corrosion) is to detect porosity that would lead to corrosion in the field.
The laboratory that runs the test can be asked to examine the surface for rust spots. At 24 hours there should be no rust spots also at 96 hours. Damaged specimens prior to testing can be identified and circled using a water proof marker, thus not included in the inspection for rust spots. Having run the first tests, future tests tested for 24 hours should be sufficient.
Generally, your customer may require a test for each lot. Lot size will have to be determined and agreed upon by both parties, and the plater. Lot sizes could also be each load in the plating tank or any number of loads you and the Air Force and plater agree upon.
For salt spray tests, if used or required by the Air Force, usually requires 168 hours to no rust spots, or an agreed number and size of spots that reject or accept the lots tested. Sorry that it seems complicated. The lab has a lot of experience evaluating the results. It would be a good idea to discuss this with one of their technicians.
There is no life expectancy specification that relate to any corrosion test. The auto and aerospace industry have test sites scattered around the US in or near industrial atmosphere locations. This is an approximate life expectancy in a variety of exposure situations.
As for testing, I suggested a thickness probe made by UPA Technology. It uses magnetic induction technology. The cost is about $2500, including the ID probe. If you use panels plated simultaneously with the good parts the ID probe would not be necessary. Then the cost drops to about $1000 to $1500. There are other methods but I think that this one is the only one I have encountered for ID measurements. For OD testing there are other magnetic instruments. There are destructive methods, but not likely useful for your applications.
*Salt spray Test
ASTM conducted salt spray test where identical panels were sent to a large number of laboratories that ran ASTM B117 salt spray testing is done. The panels were tested carefully following all the details of the procedure. The result was that there was no correlation between test cabinets. The results varied greatly
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