Letter 15756

Case Study - Hard Chrome Plating Problems - Crazing and Pin Holes [England] 

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I am a design engineer currently doing a case study at work for hard chrome plating problems as follows :-

1) crazing effect (also known as 'chicken wire' cracking and snakeskin effect)
2) pin holes

I would like to know what causes these problems and what effects they can have when pumping in oilfield applications.

The material being plated is precipitation hardening stainless steel to AISI type 630 (17 - 4 PH)which has been heat treated to NACE MR-01-75.

Please forward me any information you have.

Thanks,

Mandy Bailey
- Manchester, England

Mandy,

I would say the problem most likely lies in improper pretreatment of the base material. Otherwise it could be a result of excesive heat at the grinder operation (which you didnt mention if was caried 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.

Guillermo Marrufo
Monterrey, NL, Mexico

Guillermo,

Thanks for your reply. What would you suggest for proper pre-treatment of the base metal ? The metal component is milled and then polished on automated machines using various grades of polishing belts. Once the component is received by the chrome plater, it is then given a pre-plate polish by hand grinding tools which as you suggest can be one cause of excessive heat. With regards to pitting and pinholes, our inspectors and the plating inspectors are currently looking for these in the base metal. This has shown an improvement. I will raise the point of filtration improvements with our plating supplier. I believe that our plater does reverse etch for approximately one minute. What would you suggest for cleaning the surface prior to plating ?

Regards,

Mandy Bailey
- Manchester, England

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.)

Stefan Svetsky
- Dubnica, Slovakia

Mandy,

Most experienced platers prefer the Woods nickel strike (though there are other methods depending on facilities, etc.). This is a short forward flash of current in a solution containing 300grams/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 dont 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,

Guillermo Marrufo
Monterrey, NL, Mexico

Stefan,

Thanks for your reply. It may be that we can improve on pre-plate inspection. Before plating it is only a matter of a few hours before we dispatch the components to the platers - do you think it is still necessary to protect the component, or do you refer to protecting the bar before machining and plating ?

Guillermo,

I am not familiar with the different methods but I may be able to find out which method our plater uses. I believe that our plater does perform a short reverse etch. This is very good information, thank you for your time. I will let you know the outcome.

Best regards,

Mandy Bailey
- Manchester, England

Has anyone got any information on what causes pin-holes in HCP?

Mandy Bailey
- Manchester, England

Here is a truism that I have been lectured to by a major chrome plating process supplier several times about, Mandy: 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 piece they'll give you if you have to! After there is not a single inch of old emery cloth to be found anywhere, if the problem continues, only then will it be worth considering another possible cause :-)

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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?

Stefan Svetsky
- Dubnica, Slovakia

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 shotspeened 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 premachining, wax contamination in the solution high trivalent chrome.

Regards,

Anders Sundman     surface finishing engineer Sweden

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. Apparantly 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,

Mandy Bailey
- Manchester, England

I want to start looking at the differences in the co-efficients of linear expansion between 17-4 Ph Stainless steel and hard chrome plate. Can anybody help?

Best regards,

Mandy Bailey
- Manchester, England.

Hard chromium thermal expansion coeficient is about 4 millionths per inch per Deg-F (7.4 microns per meter per Deg Celsius) 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

Guillermo Marrufo
Monterrey, NL, Mexico

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.

Paramjit Singh
- New Delhi, INDIA

Guillermo,

I am carrying out my master thesis in materials engineering about the oxidation process of Cr at high temperatures. I have a problem searching for data about the expansion coefficient. I have seen that you know the behavior of the expansion coefficient of Cr with the temperature. Do you know the behavior of the Cr2O3 too??? How is the plot of its coefficient depending of the temperature (till 2000°C)??

Thank you very much.

Regards,

David Vilar Ferrenbach
- Barcelona

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