Acid Pickling of ASTM A351 GR: CF3M 9

[mmcastahd]

While acid pickling of castings made of grade cf8, cf8m ,cf3m, etc a kind of black deposition on the surface and inside the bore of casting is observed which gives a bad look and is tough to remove. Acid used is HNO3:HCl:H2O - 2:1:1 and dipped for approximately 30 to 40 min. please advice

A M CONTRACTOR

 

[arunmrao]

Passivation thread330-111355

What is solution annealing, pickling, and passivation?
thread723-165497

If you can place your hand on Mcguire's book" Stainless steel for design engineers",it will make an excellent reading.

 

[MikeMet]

There have been discussion on black deposits on cleaned stainless steel. There was no real determination of the problem. I have attached an article that may be of some help.

 

[mcguire]

Arunmrao, thanks for the plug, but I didn't address that problem in the book. The black "smut" after pickling cam from pickling with sulphuric acid and then insufficient or depleted nitric/hydrofluoric. I don't recall the composition of the black deposit, but it occurs when the HNO3/HF is too poor in HF and too high in Fe complex precipitates. Therefore, get new pickling solution.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[arunmrao]

Mcguire, You have done a wonderful job by bringing out the book. Hope some day,metengr,TVP,CoryPad,Ed ,either jointly or independently follow you. I hope,they are listening.

 

[mcguire]

Thanks,my friend. If you have a copy, a review on Amazon would be appreciated.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

To start with DON'T pickle stainless in HCl.

You even get the smut in the early stages of pickling in Nitric/HF.
The operator's solution is to pickle longer.
The right solution is to wash with high pressure water.

How warm is the pickle solution? This is worse in cold solutions. I like 20-25% Nitric/4-6% HF and about 130-140F.

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Plymouth Tube

 

[mcguire]

Amen,Ed. I missed the HCl. That's a real no-no.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[mmcastahd]

THANKS FOR THE RATIO EdStainless, I WILL TRY PICKLING WITH THE GIVEN RATIO AND CONDITION. ALSO AS SUGESTED BY McGuire THE 'SMUT' IS OFCOURSE Fe PRECIPITATES AS THE SHOTBLASTING IS DONE WITH CARBON STEEL SHOTS. FREQUENTLY CHANGING THE SOLUTION MIGHT HELP. I WILL LET YOU NOW THE RESULTS SOON. THANK YOU ALL OF YOU.

A M CONTRACTOR

 

[unclesyd]

There is no absolute guidelines on pickling acids which can be 10/2; 10/1; 15/3; 20/2: etc (HNO3/HF) as it all depends on what you are trying to accomplish. The higher ratio of HNO3 to HF tends to leave brighter surfaces. I never heat up the acid mixture and usually control the speed by concentrations.
Water is isn't a very good rinsing agent for removing pickling acids, I've found that a low pressure steam lance is the best approach. Knockoff what smut you can with the steam lance and then dip the part in a running water bath.
If smut is still a problem it can normally be removed by dipping the part in concentrated HNO3 at room temperature.

One thing to keep in mind is that a pickling and passivation system needs a dedicated operator. It can't be controlled by a hardcore unemployable as there is always a judgement factor to consider.

 

[EdStainless]

good comment Syd,
In general for alloys that are easy to pickle (easy to over pickle) like 430. 439, 17-4, 17-7 we would use 10-15% nitric/1% HF.
For standard 300 series alloys we use 20-25% nitric/4-5% HF.
For difficult to pickle alloys such as 6% Mo grades we go with 10% nitric/6% HF. The limit on HF is set by you permits and by the point where the bath starts to fume. HF fumes are deadly so it is good to avoid them.
If you have high Cr alloys (over 25%) a different approach is needed.
A final dip in 25% nitric is very good at cleaning the surface.
We like warm pickle baths since it helps prevent the re-precipitation of various salts back onto the product.

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Plymouth Tube