Pacifying a weld

By Lawrence (*****)

Date 07-14-2004 22:04

Could you mean Passivation of a weld?

What is Chemical Passivation?

Here is a quick excerpt from the folks at Trent Tube
http://www.trenttube.com/Trent/FAQs.htm

Chemical passivation is the chemical removal of superficial contaminants that inhibit corrosion resistance. Passivation of stainless steels is the formation of a chromium enriched surface layer that helps to protect stainless steel. Although stainless will self passivate in air at ambient temperature, the diffusion kinetics are so slow that this layer is only marginally protective. Contaminants, such as iron on the surface or manganese sulfides, can create local galvanic corrosion cells that are unable to self passivate. These are areas where pitting can initiate.

Chemical passivation is the use of a chemical or electrochemical (such as electropolishing) method to dissolve contaminates on the surface that prevent a defect free protective layer. The most commonly used solutions are oxidizing acids, such as nitric or phosphoric. Other acids, such as citric acid, have also been used successfully. Details of these solutions can be found in ASTM A380 and ASTM A967. Additionally, tests to determine the success of these treatments are included in ASTM A967.

By jer

Date 07-15-2004 01:54

Thank you for the info Lawrence. It says that stainless will self passivate itself with time, but it seems to of said areas with any iron on can eventually start to pit.

Now after welding a stainless tank can you apply the acid by just wiping on or does part need to be submerged for a period of time?

By C.Milligan Date 07-16-2004 11:59

Jer,

We operate a custom fabrication shop in the North Georgia area. We are in the middle of a job that will require passivation on a vertical surface. This is what we will be using: http://www.stellarsolutions.net/products/prodline.htm Check out #2210. This produce is compliant with ASTM a-967-01 passivation requirements. It is not as agressive as traditional Nitric acid passivaton, but if you need to treat vertical surfaces or small spots this is ideal. Just be sure to mechanically clean the weld joints to remove any slag or visible scale. It is a thin gel that can be sprayed on or brushed. I purchased a 5 gallon pail for under $100. You can get as little as 1 gallon for about $30. I was very hesitant to go from Nitric to Citric; however, the safety and cost issues made the decision easy. Our client uses a chemical inspection kit to insure that the passivation procedure was performed properly, and performs the 1% NaCl test. They were pleased with the results. If this is too much hassle, then you may consider simply sending your item to someone who performs contract passivation and cleaning. There are quite a few companies that offer this service. Most have a minimum charge, so depending on the size of your project you may call first with dimensions.

Hope this helps,

C.Milligan

By Niekie3 (***)

Date 07-18-2004 18:51

Hi jer

With welding stainless steel, there are two big issues, that lead to passivation being recommended. The first is that the high temperatures that the weld area experiences results in a layer of "burnt" oxide forming on the weld and adjacent parent metal. This layer will over time result in crevice corrosion occurring as the layer becomes a little looser.

Secondly, surface contamination such as iron tends to penetrate through the surface oxide layer (which protects the stainless steel) due to the mechanical handling which the material experiences. If this is left intact, the iron particle will experience rapid galvanic corrosion resulting in a "hole" in the protective oxide layer of the stainless steel. This further leads to the pitting corrosion that is the bane of stainless steels.

Both these problems can be solved by first "pickling", which removes any contamination, and then "passivating" which results in the restoration of the protective oxide layer. Luckily, there are preparation available that allows you to perform both these operations in a single operation.

For applications where you only want to pickle and passivate smallish areas (e.g. around the weld) there are pickling pastes available that are easy to use. For larger areas, the "liquid" variants need to be used. They are usually easily available from welding supply houses.

Regards
Niekie Jooste
Fabristruct Solutions