Case Study - Hard Chrome Plating Problems - Crazing and Pin Holes

A. Mandy,

I would say the problem most likely lies in improper pretreatment of the base material. Otherwise it could be a result of excessive heat at the grinder operation (which you didn't mention if was carried out). Both situations may lead to crack patterns. Pitting, on the other hand, can only come from previous pinholes too small to be noticed, cleanliness of surface or lack of filtration before and during process. Most chrome platers are used to reverse etch conventional steel parts then forward plate them. This cannot be done with most SS as they will become passivated during reverse and will not produce good adhesion. They have to be activated in a special highly acidic formula.

A. I would always try to inspect surface prior to chrome-plating to be sure that previously operation was right (as in process-inspection through visual inspection or microscope pictures, may be also by REM). Sometimes temporary protection was not good (by storage without preservatives - oils, VCI, etc.)

A. Mandy,

Most experienced platers prefer the Wood's nickel strike (though there are other methods depending on facilities, etc.). This is a short forward flash of current in a solution containing 300 grams/liter nickel chloride salt plus 300g/l hydrochloric acid, rest water at room temp and 2 to 10 Amps/dm2 for 1-2 minutes. Some SS perform better if first given a short reverse etch in this same solution (10-30 seconds same conditions) some other don't need or like it. This activates the chrome and nickel present in the surface and leaves a very thin layer of almost pure nickel which promotes a good adhesion for chrome. Care should be taken not to let this layer dry and passivate again (oxidize) and chrome plating must be carried out with no delay, no rinse if possible and no reverse at chrome tank.

Hope this helps but in any way let us all know the outcome.

Bye,

A. a) No, some hours shouldn't be any problem. ... Before plating it is only a matter of a few hours before ... do you think it is still necessary to protect the component, or do you refer to protecting the bar before machining and plating...

b) I think you should also make some samples with chrome-coats electroplated by various conditions to try obtain coats with defects. Also to do porosity test - number of microcracks on cm2 - could be useful (may be the stress in coat is too high).

c) Did you try to measure mechanical impurities in bath?

Q. Stefan,

b) We are currently in discussion with another plating company to see if they can replicate the 'crazing' effect.
c) We have not tried to measure the impurities in the bath ourselves but our plating company has a chemist who regularly checks the contents of the bath & makes the necessary changes.

Anders,

After chrome plating, the parts are given a polish using 'sateen' polishing wheels. The parts are not shot peened before chroming.

Ted,

After a visit to our chrome plating vendor, they are not actually using emery cloth but sateen finish polishing wheels. Apparently they are using the same wheels for rough polishing as for finish polishing, so each polishing wheel may not be at the same standard.

All,

I am currently putting together a report for my manager with causes and recommendations. It may be that we need to work closer with our plating vendor in the future. Thank you all for you help.

Best regards,

A. Hi Mandy. I'm not a chrome plater, but a truism that I have been lectured about by a major chrome plating process supplier several times about is: Take a walk around your machine shop and pick up every piece of used emery cloth that is out there. Pay your machinists a quarter for each old scrap they'll give you if you have to! Only after there isn't an inch of old emery cloth anywhere will it be worth considering another possible cause :-)

Per those experts, the "old hands" frequently keep slightly worn emery cloth on hand because it's less dusty and not as aggressive . . . but it has tiny amounts finger oils and grease which can pit your plating :-)

A. Mandy,

You don't mention anything if the part are grinding after hard chromium. A grinding machining can cause chicken wire if the operator take more than 2 µm at each cut when the grinding operation starts. Also check which types of wheel the use they should be type K or L. Did the part also be shotpeened before hard chromium operation if the hardness is over 50 HRC. I don't believe the pretreatment method is wrong. Pinholes can occurs if the part is bad pre-machining, wax contamination in the solution high trivalent chrome.

Regards,

A. Hard chromium thermal expansion coefficient is about 4 millionths per inch per Deg-F (7.4 microns per meter per °C) at room temp and increases with temperature (50% at 1,000 °C). 17-4ph steel is slightly higher (around 5 english, 9 metric) G. Marrufo-Mexico

A. As a very effective means of activating SS before Plating, try a method, what we generally refer as 'Hydrogen Flushing', i.e., raise the current in the Plating bath very slowly only to a limit, where the Hydrogen Gassing Starts but the Plating does not. This can be easily set, i.e., the current when the first Hydrogen Gas bubbles become visible. Let this be ON for a minute and then slowly raise the current to the final Current Density. This helps to a great extent in providing a good adhesion over SS.

A. Mandy,

I think that you must keep attention during 17-4PH steel cleaning process. This clean-up process is widely used!

I'm not sure, maybe it's inside AMS2460 - Plating, Chromium

We'd this problem pattern about one year ago and we noticed that it was generated during clean up process.

We are using a Sulfuric Acid and Hydrofluoric Acid-dip mix bath at room temperature for 3 min.

A. Mandy,

I would like to give information for relationship between hardness and Chrome plating.
17-4 PH has high hardness, so it has high surface stress also.
Some cases are cracked during ground condition as high hardness with severe grinding.
During Chrome plating, Hydrogen ion penetrated to 17-4 PH work.

Hydrogen ions combine to hydrogen gas in work surface, so the gas volume makes to high pressure with tensile stress.

Finally we get the cracked work piece by grinding crack or hydrogen embrittlement crack.
It is little difficult distinction between grinding crack or hydrogen crack.

However we can distinguish with removal plating and etching by nital (nitric acid 3% + alcohol).
If the removed surface has surface crack, the is evidence for pre-crack, grinding crack.

Simon

A. Hi, Dhananjay.

I don't know if local libraries or local bookstores have it, but Guffie's "Hard Chromium Plating".
is the one book that I am aware of that covers how to do hard chrome plating and resolve problems. Good luck.

Regards,

A. Hi Vishal. I can appreciate your frustration that successful hard chrome plating can be very hard and you need to know what is wrong ... but your only options are either to retain a chrome plating consultant to help identify and fix the problems, or to learn enough to effectively troubleshoot the problem yourself. Although people would like to help you, they spent years learning how to chrome plate well, and are unlikely to be able to answer the question of what you are doing wrong that is causing partial coverage in a paragraph or two. But send photos of the problem, and describe what you are achieving and what you are not, and they can at least try. Good luck.

Regards,

A. How are you cleaning the roll?
How are you activating the roll?
What is your anode material?
What is the tank temp?
What voltage and current density are you using?
What is the anode to cathode distance?
What is your chrome to sulphate ratio?

A. A quick test for some of the issues your having is a specific gravity test using a common hydrometer. Look up the specific gravity of your plating bath composition then check the solution at the specified temperature and any reading higher than it should be is excess metals and/or trivalent chrome. Once this is known most (but not all) of these can be reduced using a porous pot (look it up) as well as dummying the bath to lower the trivalent chrome. The general rule is 1 oz per gallon MAX other metals (iron, copper, zinc, aluminum, etc.).