Corrosion Pitting Prevention on 304/316 SS 2

[Mechwill]

Hi all,

My company has come across a situation of corrosion pitting inside of the stainless steel tank right along the weld. The welding materials is 316L SS welding rod to weld 304 SS sheetmetal.

The application of the tank is to heat up different liquids in the tank and transfers heat for hot water or hot air use. The liquids that needs to be heated up are antifreeze or domestic water (city water) at high temperature of 200 ºF.

I have done some research of preventing the pitting corrosion. The methods that I have come across are as following:

[li]Selected the materials with known resistance[/li]
Currently, we are using 304SS and 316SS will be the next selection due to higher corrosive resistance. However, the researches that I have conducted state that the 316SS will last 6 months to 1 year longer compared with 304SS. Furthermore, the price is 78% higher.

[li]Control pH, chloride concentration and temperature[/li]
Due to our application, it is impossible to avoid that.

[li]Cathodic protection and/or Anodic Protection[/li]
I am not sure about this method since there are other materials welded onto the SS sheet metal.

[li]Use higher alloys (ASTM G48) for increased resistance to pitting corrosion[/li]
As it is stated in the first method.


Another way is to prevent the pitting corrosion is to have a steel coating with porcelain or epoxy. However, it worries me of the high temperature antifreeze in the tank.


All the suggestions are welcome and appreciated. Once again, thank you for your inputs

 

[EdStainless]

You need to find out if the pits started because the welds were not clean (heat tint), or if they just don't have enough corrosion resistance.
If they are pitting from weld oxide (heat tint) then it can be prevented. Using good weld shielding and then a pickling paste to clean them would do the job. Do not let anyone grind them to clean them, this does not work.
You could try to salvage the current tank (if the pits are not too deep) by grinding them out using a fresh grinding wheel. Then cleaning the ground material with a pickling paste. My hunch is that some of the pits are over half way through if you can see them. This tank is history.

Another alloy option is a lean duplex stainless steel. Alloys with names such as 2101, 2003, 2202 are all candidates. They have pitting resistance similar to 316 (or a little higher), the welds (use 2209 filler) do not loose as much corrosion resistance as in 304/316, and they are much stronger so you may be able to use thinner material (if hte design allows).

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Mechwill]

Hi EdStainless,

Thanks for your reply and input.

One of the tanks has returned to us due to the problem of pitting corrosion. So, the pits have already started. We didn't have this problem before until recently when we change the material. So your guess is as good as mine. Probably the poor quality of stainless steel.

As you state, the pits start at on the line of the heat tint. I will look it up the suggestion of using good weld shielding and pickling paste. Pardon me for lack of knowledge on welding manufacturing processes. Is there a general procedure of applying the pickling paste?

I will also consider the option of lean duplex stainless steel. Not sure the price is in the acceptable range for 2101, 2003, 2202.


Once again, thank you for your input and suggestion.

 

[SJones]

When you say that 'one of the tanks has returned to us,' does this imply that you are not the end user with control over the quality of the tank contents?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Mechwill]

We build the tank in the shop and sell the unit which has the tank in it. We are not the end user but manufacturer

 

[OGMetEngr]

EdStainless is right on the money on this one. Make sure you are using 304L (not 304) and clean up the welds with pickling paste, this (along with the other direction from Ed) should take care of any pitting at the heat tinted area. If more corrosion resistance is needed, 316L or duplex is the way to go.

 

[Mechwill]

I would like to apologize to you all for not stating one important element of the situation. The pitting corrosion occurs on the HAV at the INSIDE of the tank where the antifreeze and high temperature and all the stuffs interact with each other. The outside of the tank along the weld is perfectly fine. I did some research on pickling and the method for applying it. It does seem like a nice solution. However, is there a pickling paste that can withstand such high temperature as 200 F and resists high corrosive fluids such as antifreeze?

Thanks

 

[SJones]

The pickling past residue is removed before the item is put into service. Have you not wondered whether the pitting has been caused by what the tank user has been putting into it?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Mechwill]

Hi SJones,

As far as we know, we recommend antifreeze with mixture of 50/50 solution with water. What the customer claimed is that they have put different brand of antifreeze with mixture of water. However, once the product is out of the door. There is no real way to find out what liquid they actually put in the tank.

The pickling paste basically clean the surface of the welds by acid treatment to remove high temperature scale produced in welding. Will it work better to use pickling paste first and then passivation?

 

[SJones]

http://www.nickelinstitute.org/en/T...ngStainlessSteelSurfaceTreatments_10068_.aspx

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Mechwill]

Hi SJones,

This is such a wonderful reading and reference. Thank you for the information.

 

[Mechwill]

Hi All,

I have one more quick question about the pickling and passivation.

I understand it is almost a standard to do the pickling before passivation since the acid that they use in pickling is hydrofluoric acid (HCl) mixture, and the passivation is usually diluted nitric or citric acid (H3C6H5O7 ). I also understand the purpose of picking and passivation.

However, is it possible to do the passivation and the pickling at the same time in one step? does that mean there will be a different acid for both passivation and pickling? If there is, what would be the name of this kind of solution?


Thanks

 

[SJones]

It depends upon which definition of "passivation" you are using. If you are using the ASTM A380 definition, then it will be the removal of surface exogenous iron, which pickling will also do. If you are using the ASTM G193 definition - the acceleration of surface film formation, and its subsequent build up, then it is clear that pickling and passivation can never be performed together, since the pickling solution removes the protective layer. Take a look at the plea from the technical editor on the front of the Outokumpu periodical attached.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Mechwill]

SJones,

Once again, another amazing information to clarify my question to passivation. I did not know that the passivation has other definition other than forming a surface film. What I have found is that the traditional method of passivation is to have a film on the surface of the metal. I will look into both ASTM standards and the attachment that you provide. Once again, thanks for the clarification.