Zinc Phosphate coating problems & issues; Q&A's

A. Hi Waqar. According to Biestek & Weber the short answer to why phosphates attach/adhere well is that they are formed "at the very reaction site". In other words, there is not a reaction out in the bulk of the solution which causes the precipitation, but a conversion reaction right at micro-sites on the surface of the metal.

I don't have personal experience to be able to tell you what makes for evenness of deposit and scratch resistance, but if you can get access to the book, it's very good at the theoretical level. Or you can read this thread and thread 13366 for a lot of readers' solutions to zinc phosphating issues.

Luck & Regards,

A. Hi,

Let me answer a step at a time:

a. Method of getting a uniform phosphate coating. Presumably you are using HCl acid; ensure sufficient time for complete descaling; tease the coil out on the hook to open the coil up to the solutions; immersion rinse in water then power wash the coil with high pressure sprays. This will remove residual acid and smut. Failure to rinse properly will contaminate the phosphate and slow the process up. Give 10 min process time in the phosphate minimum.

b. Similarly, method of getting uniform stearate coating. Alkalinity of the soap bath must be controlled. More alkaline soap will give heavier soap weights but will also dissolve more phosphate. A rinse in Borax [affil. link to info/product on Amazon] solution before soaping will protect the soap bath from acidic carryover.

c. Sometimes we find that after the phosphate coating and stearate coating the material develops rust spots.How to prevent this? May be due to poor storage conditions; poor rinsing; poor soap coating; incomplete descaling.

d. Above Coated material at times gives sound while wire drawing to a smaller size.What could be the reason? Note : Phosphating is done at 65 °C and Stearate coating is done at a temp of 65-75 °C. Noise will be failure of the lubricant layer and will cause premature wear of the dies.

Hope this helps,

A. If this condition appeared abruptly, my guess is that the screws are of a higher carbon content or are heat treated to 2-4 Rc harder.

Prebake is one help. Longer post plate bake is another. Probably the biggest is getting the screws into the oven faster after plating. All three together should make your problem less.

A. CHRIS,

IF YOUR PART IS ROCKWELL 38C OR HIGHER IN HARDNESS, THEN THE COATING SHOULD BE MECHANICAL ZINC WITH A DICHROMATE FINISH. BE CAREFUL USING PHOSPHATE COATINGS, WE ARE PRESENTLY DOING A PHOSPHATE - BAKE - OIL FOR A CHRYSLER UNDER THE HOOD SPRING AND THE SPEC CLEARLY STATES NO ACID PICKLE.

CONTACT GORD ALLISON AT Macdermid IN WATERBURY CONN.

HE CAN SEND YOU A 5 YEAR CORROSION STUDY SHOWING ALL ZINC FINISHES.

A. Chris,

Hydrogen embrittlement(HE) is associated with zinc plating. A number of processes have been developed to minimize and even eliminate hydrogen embrittlement. Acid chloride zinc plating process is able to reduce the HE problem to some degree. Most importantly, baking is strongly recommended to immediately follow the zinc plating process in order to eliminate the risk of HE. I suppose your suppliers should be able to solve the problem related to plating processes.

You mentioned that you are intending to replace zinc plating with zinc phosphating. Of course, zinc phosphating can provide the limited corrosion protection to steel substrates. However, zinc phosphating is generally used as a paint base instead of a major corrosion protection measure. This is because zinc phosphating coatings cannot provide the corrosion resistance comparable to that resulting from zinc plating with the chromate conversion coatings. Zinc plating has a sacrificial function in protecting steel substrates while zinc phosphating only acts as a protective barrier with some micro pores.

Ling

A. Chasing sporadic problems is a pain in the butt. My first thought is that maybe you're getting parts with some inclusions in plating, or stress cracks, or maybe just a soft spot in the plating. Any of these things will cause the coating to form differently in those areas.

Try flexing a part to from stress cracks, smack one with a wire brush to from dimples in the surface, and maybe abrade one to see if the abraded area looks like the "nubbing" problem.

Another thought is that your cleaning may not be up to snuff, but it sounds like you've addressed that, and chemfil products will do the job. Maybe check the location on the rack of the problem parts to see if it coincides with any apparent dripping or running of solution that could carry trash onto the parts. Good luck.

The Problem has been eliminated. After changing jobs, to work in a zinc plating plant, Tai and myself have been better able to troubleshoot the problem. We hypothesized that there were catalysts trapped in the matrix of the zinc that caused the phosphating reaction to be accelerated. The highest concentrations of these "catalysts" would exhibit the nubbing effect.

We then took a look at a very long list of possible substitute chemicals which can act as a catalyst in the phosphating, and found that an extensive number of different organics can behave in such a manner. And as we were aware, zinc plating uses many organic components to achieve the desired plating properties.

We then decided to test the zinc phosphated part for phosphate coating weight and found them to be very high. Tai then tried to significantly reduce the immersion time in the phosphate bath, resulting an an acceptable coating weight with no nubbing. So, it seams that this reduced time is enough to achieve desired coating weights without promoting the localized excessive phosphate crystal growth.

Regards,

A. Let me try to help you :

Your process is not the optimum one :

a.First stage: Instead of hot emulsifying cleaner replace to Alkaline colder cleaner. There are a lot in the market .
b.Second stage: Phosphoric Acid, cannot be alone component in the bath, the bath must have surfactants, I suggest as in the previous stage, buy yourself one of the commercial products in the market.
C. The two next stages must be commercial products too, not self formula products .
D. The problem might be the final rinse stage, there is no need to heat this stage, if you heat it to such a high temperature the water must be Deionized.

Shortly : Try to use well proved commercial products I'm sure you have several suppliers in your area. Do not heat the last final rinse it is not necessary at all.

A. Because your phosphate bath too hot, I suggest 140 °F max
and don't add anything (soda ash) in rinse tank, just fresh water, because your's pre-treatment line has a post rinse (Cr rinse).

A. I have been told by the manufacturer that zinc phosphate will destroy the carbon probe and must be removed before heat treating. If you are not running a carbon probe in your furnace I am not aware of any other problems.