Zinc-plate appearance problems (white, powdery, rough surface)

Q. Came across parts after batch zinc-plating in acid-chloride that exhibit a white, powdery, streaky appearance (Parts are flat ~16 x 16 w/ moderate forming). When compared to "good" batches, "bad" batches seem rougher in texture (along with a overall lower reflectivity). Subsequent rubbing of the finished part surface with your hand exaggerates the powdery condition and leaves a white residue. You can clean off an area of a "bad" part with an acetone [adv: item on eBay & Amazon]-soaked rag which removes the white residue and the zinc coating remains intact.   

It's questionable whether the substrate and/or plating process contains the key variables for this problem.  Batch process utilizes two rounds of cleaning w/ de-ionized water and finally plating in a acid-chloride bath. Parts and finally a clear chromate conversion coating to passivate. 

Anyone ever see this condition and have any ideas? Thought maybe surface roughness of the substrate and/or clear chromate was the culprit.

Thanks for the help.

A. Hello Chris. You say you 'came across' these, which leaves us with the questions of whether they are new or old and whether you are working in the plating shop or these parts were shipped to you, etc. My first guess is it's boric acid. Some plating shops (low tech, non-spec work) will on occasion hang a bag of boric acid in a plating tank and almost immediately start plating, with result that crystals or powder form on the plated parts.

My second guess equals your first guess that it's a clear chromate problem.

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

Q. Ted, thank you for the quick response. To help clarify my original inquiry, we are currently supplying low-carbon 1006/1008 CRSH to a customer who stamps parts and uses their own plater. Our customer and their plater are quite frustrated and I can understand they would want to question the substrate properties (ladle chem., rust-preventative oil, etc..). They've requested we be a main player in the analysis. I've begun working with their plater (via phone) to help them resolve and to this point have only received "good" and "bad" parts along with substrate ID(yet to do SEM/EDS on the powder). Info. gathered thus far leads me to believe there are more problems to worry about than the substrate.

Info gathered:

-All supplied substrate is confirmed to be supplied with the same rust-preventative oil (Quaker Ferrocote 61).

-Supplied substrate is Al-killed 1006/1008 w/ very low residual Si and similar chemistry, however a few shipments had higher levels of Cu (.11% vs .05%). Would higher Cu levels in the steel makeup be an issue?

-Supplied substrate had surface profilometer values measured in microinches of ~30-50 which is seen as typical for most mills at the same gauge (.040"-.050"). Maybe higher surface roughness holds excess solutions/oils through cleaning/plating/chromating?

-Plater uses alkaline cleaning solution w/ two rounds of rinses and plates out a .0002 - .0005 flash zinc in acid chloride w/ a final trivalent coat(clear chromate).

-Plater reports that they will run a batch of parts that will exhibit this condition IMMEDIATELY AFTER THE PLATING PROCESS (apparently within seconds) and run another soon after, apparently under the same conditions with another batch of similar steel (same grade, etc..) without issues (nice bright, uniform zinc). Keep in mind I haven't determined if "immediately" means as soon as it comes out of the acid chloride tank or right after the chromate is applied (which I will obviously need to investigate). Could you help me understand the steps between plating and chromating and between chromating and finished product?

-There's apparently no handling involved, as the plater uses a circular rack which can hold ~80 of these parts at one time.

-The condition seen on these parts is strikingly similar to the bottom left photo of letter 18213, however, the entire surface seems to be very very light white in color (powdery and streaky with a rougher surface). It becomes even more exaggerated though when touched or rubbed with any applied pressure.

-The plater has been relying on what has been referred to as a "time-saver" which we are told is a belt-sander that is used on the steel prior to plating. This works well for them and is a large part why they are questioning substrate.

Being more process metallurgy oriented with only some electroplating knowledge (and knowing not to overstep my bounds), I'm looking for help from the finishing community to direct the right questions to the plater and help set up experiments to determine root cause.

Thanks again for the input

A. CAUSES FOR BURNED DEPOSIT,

1.IN THE ABSENCE OF ADDITION AGENTS,THE DEPOSITS ARE COARSE-GRAINED AND SHOW NO REFLECTIVITY,QUALITY IS VERY POOR AND MOST OF THE DEPOSIT CAN ACTUALLY BE RUBBED OFF BY APPLYING ONLY A LIGHT ABRASION.

2.LOW ZINC METAL CONTENT.

3.TOO HIGH CURRENT DENSITY.

4.LOW OPERATING TEMP.

5.LOW BORIC ACID.

ETC,.

ROUGHNESS:- EXCESSIVE ANODE SLUDGE,TORN ANODE BAGS. REDUCE AIR AGITATION NEAR ANODE BAGS.

Ajay Raina
Ludhiana, Punjab, India

Q. Good day,
I have a current customer complaint due to a surface finish on one of our part numbers that they define as "rusty". Before we ship the parts to our customer this condition is not present. Parts normal transportation is by sea (I go ignore if that is a factor for this "rust" to appear) but the customer claims that not all the parts have this condition so we would like to know what to look for at with our supplier that plates the part and focus on the factors that could produce these "physical appearance or rust". I appreciate your help.

A. Dear Monica, You must focus on your plating supplier and the packing of your parts. You must seal the parts (by putting them in bags, covering them in nylon films, etc) to assure that the transportation is not an issue. If this was done properly, then you must see if the plating process was done well... When I see this photograph I see two problems: - Plate peeling (bad adhesion of the zinc plate) - Dull plating (not bright plate, this means almost certainly that the corrosion resistance is not good) I suggest you attack the problem (if it is) of the packing first, as it is the easiest. Then, you should talk to your plating supplier to improve the plating quality (you may do well if you inspect the parts that you have to ship, and if you see any defects, take photographs and take the parts to show them to your supplier). Hope this is clear enough, best regards and good luck! Daniel Montañés - Cañuelas, Buenos Aires, Argentina A. Monica, The customer could be correct. You might try putting some parts out on the roof for a week or two and see what happens and leave some in salty water, tests like that. You can also heat some up and rub some and check to be sure the parts don't look like the customer is reporting. If you can or cannot duplicate the results, the first thing to do would be to check the work instructions and be sure the process is being followed and that the equipment is in fine condition. If the equipment is in very bad shape and there are no instructions then I would start there. Assuming you have really nice equipment and people following instructions, you can try tweaking your process a little bit by running fewer parts, adding more process time and other tricks to make a thicker coating. It could be that you're a little flaca. Buena suerte. Blake Kneedler Feather Hollow Eng. - Stockton, California

Thank you very much!! I already started questioning this. I appreciate the feedback.

Q. Dear experts,

We are doing zinc plating (barrel plating) with trivalent white, hexavalent yellow passivation for our machined parts. After plating, these parts are packed in polythene bags and put inside cartons in a closed atmosphere. Some parts are not sold immediately were kept at our warehouse for one year. These parts were re inspected after one year and found passivation fade out and white patches. Please help to resolve the issue.

Hi Dinesh. They probably weren't dry.

Regards,

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

Q. I run an alkaline non-cyanide zinc electroplating line and we are having issues with white patches on our metal screws. Our plating solution is kept at 35 °C; our rinse tanks are cleaned regularly; and we use a trivalent blue chromate. Currently our caustic is at 142 g/l and our zinc is a 23 g/l

Q. Dear Expert,

I'm facing cloudy white patches/stains surface on my rack plating. The process is cyanide zinc electroplating. The surface is smooth but came out with those stains as shown in figure below:



Anyone ever see this condition and have any ideas? What are the potential cost and solutions? Please help. Thank you.

A. Hi. What is that thing? ... and what is the substrate material and/or fabrication method? Is it a casting or forging?

It looks like there is a white haze, and even a yellowed area, and the plating looks awfully bright for cyanide zinc -- so maybe it's too much brightener in your zinc plating, which is preventing proper chromating. But I also see deep scratches or cracks and a couple of deep dings, which might indicate that the part is just not sufficiently mechanically prepared for proper plating rather than something specific that you are doing wrong. I appreciate you sending the picture but if you could please tell us what it is and what it's made of it would be helpful. Thanks!

Luck & Regards,

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

Q. Some particular customer's Machined parts pre-parts after alkaline zinc plating: white patch coming after plating. In spite of pre-cleaning and double anodic and double picking also is not resolved.

Alkaline zinc plating 8 - 12 microns with Trivalent yellow passivation

A. Hi Chellappan. A picture is worth a thousand words. Please try to provide pics of the finished product and the readers may be able to help better. Unfortunately "white patches" is rather vague and could have lots of causes from white rust, to burning, to water staining, to pitting, to brightener haze, to poor chromating. Read on though, where you will see many kinds of white patches resulting from many causes.

Luck & Regards,

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

Thank you Sir,
Now I found the root cause. White patch happened due to some thick oil applied after machining by customer and there is huge time delay between machining and plating such that oil got dry and is not removed in regular pre-cleaning method. If we do flash plating and do replating then it comes out good. We discussed already with the customer to use the right RP oil.