plating, anodizing, & finishing Q&As since 1989
Phosphating Concerns With Acid Pickling
Q. Phosphating criterial needs: Our company is currently updating our phosphate line. We have purchased a much larger system that will allow us to zinc phosphate all of our product. We produce mainly spring products. With this system it has been brought to my attention that a pickling agent (HCl) would be a wise way of processing it. I am leery of introducing a new chemical into our process without knowing the advantages vs. the disadvantages. Here are my list of questions: First, how much extra mg/ft2 will this allow for a typical coating using the pickle process as opposed to not? Second, what about hydrogen embrittlement and what concerns should we be aware of? What percent solution should one use, and is HCl the best acid for our process? Are there any oils that can produce a far better salt spray without the use of a pickle? I have heard of a hot oil application that would allow us not to have to spin the product dry. Is that true? Is the pickle a common practice amongst most phosphating coaters? How many rinse stages would be appropriate after the pickle?
To answer all, and if any of these questions for our better understanding of this process would be most appreciated. Thank you very much for your time. Sincerely yours,William Gipson
- Grand Haven, Michigan, USA
A. Hi - you're right to be wary of changes. Acid pickling will indeed increase coating weight by up to 50%, dependent upon process type used.
HCl is cheap but causes corrosion in the vicinity due to its fumes. I prefer phosphoric acid pickles. In any event the use of phosphating will render the use of post baking mandatory to reduce hydrogen embrittlement. This will need to be done before oiling.
If you want a "clean" oiling system I suggest using a hot emulsified oil which will flash dry. 120 hours salt spray should be achievable. RRoger Bridger
Walterisation UK Ltd - Croydon, UK
How much Pickling time before phosphating?
Q. At my job we manufacture couplings for the oil industry. After the couplings are machined and go through NDT, we either zinc phosphate or manganese phosphate the couplings based on customer specifications. The couplings come in all different grade from K55 all the way up to Q125.
For the material grades that have a yield strength greater than 95KSI, we are required to pickle them per our standards. The pickling solution we use is a HCL/water immersion bath with a concentration of 16-18% by volume. We do not use an inhibitor in our pickling solution.
My question is how long is too long to pickle our couplings? The pickling times range from 5 minutes all the way up to 15 minutes sometimes. The pickling tank holds about 500 gallons and anywhere from 500-800 lbs of steel is processed at one time.
Manufacturing - Houston, Texas, USA
August 11, 2014
A. The variables are time,temperature, concentration & agitation.
All must be considered to calculate an effective process for a given range of soils.
Hope this helps,
Trainer - Newcastle, NSW Australia
August 22, 2014
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