Hard chrome plating vs. salt spray hours requirement

A. Hi Carlos
Here are few thoughts. I'm not sure if they will solve your problem, but some points in your post caught my attention.
1. Your steel rod is very hard. Probably it has to be stress relieved after the grinding and prior to plating.
2. If you etch in the same plating bath, there is a chance for Iron accumulation. It make sense to test for Fe in the solution. Gross recommendation: Sum of Iron + Copper + Cr trivalent all together are below 7.5g/L (1oz/gal).
3. After the Chrome plating, you do not mention Hydrogen Embrittlement bake?..
4. If you Chrome is softer after the grinding - there is a chance you overheat the Chrome at some point of time (during the grinding?)
Just my 2 cents (actually 4 cents...)

A. Hi Carlos. This thread is starting to drift pretty far from salt spray hours, but what can we do :-)

Electrocleaning is a very common process step prior to plating. When electrocleaning, the workpieces can be connected to either the positive pole (anodic cleaning) or the negative pole (cathodic cleaning). Oxygen will be generated at the anodic pole anodic pole, and hydrogen will be generated at the cathodic pole. Anodic cleaning is more common: twice as much hydrogen is produced as oxygen, so the cleaning is often more efficient. But when you must avoid exposure to hydrogen, only anodic cleaning should be used.

Luck & Regards,

A. Hi Carlos. My answer was in response to your question about what anodic cleaning is and what it has to do with hydrogen embrittlement. Sorry, I don't think anodic cleaning would offer any improvement in salt spray hours.

Luck & Regards,

A. The anti-corrosion resistance of chrome coatings is directly dependent on their porosity, due to the fact that the potential difference of the chromium - steel pair is quite large (for example, about 0.5 V in a 3% sodium chloride solution), the base metal under the pores in the coating is subjected to intense pitting corrosion.
Combined two-layer chromium plating is used as standard for corrosion protection. Milk chrome, which has a very low porosity, and shiny hard chrome are successively applied to the parts. The first layer of milky chromium 20 microns thick is deposited at a current density of 30 A / dm2 and a temperature of 70 °C, the second layer of shiny chromium 20-30 microns thick is deposited at a current density of 55 A / dm2 and a temperature of 50 °C. This combination allows simultaneous protection against corrosion and mechanical wear, but reduces the productivity of the process.

Thanks Nik! I'll definitely try this out. I'll let you know.

Thanks Nik, I was unaware of even the existence such a process, although I do know of nickel plating under hard chrome to deal with the porosity issue.

Luck & Regards,

A. Perhaps you have a different name for this process (I attach a photo of such a two-layer coating). In Russia, this method of increasing corrosion resistance is very common. However, not all tank materials are resistant to 70 °C.

Nik Erm
- Nizhniy Novgorod, Russia

A. Nice micrograph. Some Americans may be familiar with this under this name or a different name, but again I personally wasn't -- so thanks again.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

! It is strange that in your country there are no additives in the chromium electrolyte that would allow avoiding hydrogenation of the coating and substrate, thereby avoiding the appearance of cracks and pores responsible for corrosion, as well as chips and flaking of the coating at large thicknesses. This is common in Russia. Sometimes, customers require 600 hours of NSS. In the photo there is a plate with a chrome coating of 150 microns on each side, the microhardness of the coating is 1050HV, bent three times by 180 degrees, without delamination.

A. Hi Carlos. While waiting for Nik to get back to you, you might want to review this patent application for double-layer chrome plating.
https://patentimages.storage.googleapis.com/a4/9c/6b/84b3bef3acc559/EP1215304A1.pdf

I don't know the actual current patent situation, and what you may be prohibited from doing, but this patent application is interesting and easy reading. An alternative which is apparently widely used in India on motorcycle components is nickel plating underneath the hard chrome layer.

Luck & Regards,

? Carlos, did you manage to try the proposed method and get some results?

A. After chrome plating there is not any grinding process.

Many problems occur when grinding after plating:
1) micro crack
2) heating
3) peel off, etc.

It's all Reasons of sst failure,
Also work on grinding process before plating:
1) coolant
2) grinding depth cut
3) grinding cutting speed

A. Hi, Kiran. Hard chrome plating is not particularly corrosion resistant because it tends to be cracked or porous. 96 hours is a more typical requirement, but 20 microns is minimal and I think it offers virtually zero chance of resisting 200 hours. Why is the component hard chrome plated? What is it made of? Why isn't it nickel-chrome plated? A lot more info is needed. Good luck.

Regards,

A. Hi, Kiran. Unfortunately we'll have to wait for input from someone who does similar parts and has some specific experience for a more definitive answer, but my general feel is that hard chrome plated parts of this general range of thicknesses will not resist a salt spray test.

Regards,

A. Dear Kiran,

One way to increase the corrosion resistance to 48 hours CASS is to Duplex Nickel plate the parts with 15 microns of the double layer and then go for Hard Chrome. All motorcycle front forks are plated this way.

A. Hi,

I don't think you can achieve 200 hour in duplex together with electrolytic nickel in 15 µm and 5 µm hard chrome. Maybe together with Electroless nickel you can achieve about 100 hour. I also don't think you can get it together with PR hard chromium. But PR together with a first layer of Electroless nickel can give you a better corrosion protection.

A. Dear Sir,

I think Mr. Kiran is referring to Neutral Salt spray Test. It can be achieved with Nickel undercoat, preferably of sulfur-free Semi bright Nickel layer which can help getting improved corrosion resistance.

First, we need to confirm what test is specified in specification. There are three versions of Accelerated corrosion tests, CASS being most severe of them and is generally specified for Automotive components.

Regards,

A. I did a quick test with a couple mils of chrome on a test panel in a salt spray chamber, it took around 48 hours for corrosion to come up through the microcracks.

If you want corrosion protection, put some nickel down beneath the chrome. As long as the nickel is not porous, you should be able to pass however many hours you want in the salt spray.

You could try thin dense chrome below the hard chrome, but I have never done it so I can't promise anything.

"Trouble in Your Tank: Handbook for Solving plating Problems"
by Larry Durney

on Amazon

(currently UTL
on AbeBooks & eBay)

(affil links)

A. Hi Jayapraksh. You are unaware of anything at all changing? And the appearance is still acceptable and identical?

As Larry Durney tells us in "Trouble in Your Tank" .
it will probably prove much easier to really study and work through your history to figure out what changed, than to try to figure out what changed from the fact that it's not working anymore because -- as your question implies -- there are so many possible causes.

But if you want to start tabulating stuff, from chrome concentration to sulphate ratio, operating temperature, current density, certified analysis of steel rods, plating thickness, etc., readers can give a try to figuring out what is out of spec. My bet is you stopped doing something 90 days ago and the resultant failures are building up, rather than switching on like a light switch. Keep us posted please.

Still, 500 hrs NSS from chrome without an undercoat sounds rather unbelievable, so you might want to re-audit those earlier results.

Regards,