Chloride SCC with All Stainless HX construction

[mjpetrag]

I saw this on a website and wanted to know how true this is.

"To take another example of chloride SCC of austenitic stainless steels, tube and shell heat exchangers are frequently constructed using stainless steel tubes (since these must be thin-walled and corrosion cannot be tolerated) with carbon steel tube plates and shell (since these can be made much thicker to provide a corrosion allowance). Chloride SCC is rarely experienced with this construction. However, it is quite common for an enthusiastic engineer to decide that the replacement heat exchanger should use an “all-stainless” construction to avoid the unsightly corrosion of the carbon steel. The result is frequently a rapid failure of the heat exchanger by SCC or pitting corrosion. This is because the carbon steel adopts a relatively low electrode potential that is well below that required to cause SCC or pitting of austenitic stainless steel, which is thereby protected. When the all-stainless construction is adopted, this unintentional electrochemical protection is lost and failure occurs."



I ask since some of the heat exchangers we have here are all stainless construction and have seen many failures due to Chloride stress corrosion cracking. Can anyone provide technical information for this?

-Mike

 

[MikeMet]

I don't agree with what was posted from the other site. It has been my experience if you have sufficent conditions, temperature, chloride content and stress, you can get ClSCC of stainless steel in a heat exchanger with all the other parts being carbon steel. The stainless steel strip used to make tubes is typically solution annealed, and then the tubes themselves, are almost always solution annealed after welding which helps remove stresses in those areas not rolled into the tube sheet. So one major component necessary for ClSCC is removed, which can greatly reduce the possibility of SCC.

However, there are always things that can happen during the exchanger fabrication process to reinduce stresses. We just had ClSCC on the OD of Type 304L tubes at mechanical scratches induced by pushing the tubes through the tube sheet and baffles. I have very nice transgranular cracks in the bottom of the gouges that were many inches in length.

I'm sure you will get many comments on this one and there can be a million case histories of ClSCC of stainless steel tubes in a carbon steel shell bundle. Bottom line, don't put stainless steel tubes in an environment where ClSCC may be a problem or ensure you don't induce stresses into the tubing if the process can cause SCC.

 

[EdStainless]

I am more or less with Mike on this.
While CSCC requires active corrosion to initiate it would be rare for CS to offer enough galvanic potential to suppress the corrosion.
Evaluating the design for unintended stresses such as from thermal expansion or vibration is important.
It is also critical to limit the residual tensile stress in the tubing. Even properly made and well annealed SS tubing can have residual tensile stresses that are more than 50% of the yield strength. Special processing is required to achieve reliably low residual stress levels.

However, if you plan on relying on controlling the stress or Cl in order to avoid CSCC then you are using the wrong alloy. These are inexact controls and can only be relied on to provide a usable safety margin, not protection.

This is one reason why ferritic and duplex stainless steels are often selected.

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Plymouth Tube

 

[strider6]

cathodic protection can reduce the risk of CSCC but as Ed said i'll not rely on this.. i'll make a different material selection and select a material that is fully resistant to SCC in that operating conditions

S

Corrosion Prevention & Corrosion Control

 

[mjpetrag]

Thanks for the replies. This confirms what I thought to be a poorly worded statement.

-Mike

 

[unclesyd]

We ran over 100 vertical SS Hx's the majority of these bundles were of the external floating head type, excuse the lack of proper nomenclature, with both SS and CS shells in CTW where we mitigated the problem of CL SCC by using tubesheet vents on each and every Hx. The key point was that these vents had to be operating to keep the backside of the top tubesheet wet and flushed. If these vents were closed off you would almost be guaranteed to loose the bundle in a few days.

The loss of Chromates forced us into some very expensive material changes with several trial ands tribulations due to unresolvable MIC problems.