SCC of 304/316SS 1

[UKCats]

Can normal levels of chlorine in potable City Water cause SCC in 304 or 316 Stainless steels. It is my understanding that the presents of any halogen (Cl, F, Br, etc.) at any concentration, below say 100F, is a non-issue and somewhere between 150-200F, it becomes a concern. However, I'm not sure at the low concentrations of Cl in city water, that there is an issue, even at elevated temperatures.

 

[rustbuster1]

Chloride-induced SCC of 304SS is depends on temperature and on the chloride concentration - as you recognize. There is no risk of CSCC in 304SS or 316SS at temperatures below 120F in domestic water service. That does not mean that MIC cannot still cause pitting in some conditions, but since the whole point of the chlorination is to disinfect, even the chances of MIC should be negligible.

 

[UKCats]

Is there a concern with Chloride-induced SCC of 304SS above 120F in domestic water service?? If so, would the most appropriate manner in treating this is using an ion-exchanger (i.e. resin) of some type??

 

[TBP]

The "Nalco Water Handbook" has information on this in the second edition. It indicates that in some desalination plants, SS has not experienced any failures in spite of chloride concentrations exceeding 30,000 mg/L, if properly stress relieved, and kept free of deposits.

The Nalco book says (in part): "When this type of failure was first experienced, it was thought that chloride concentrations as low as 50 - 100 mg/L were responsible. But experience indicates that the concentration of chloride in the water is not the critical factor. The main factor is the existance of conditions that allow chloride cells to develop."

I know of a number of all SS heat exchangers in pool/spa/domestic water service being heated by steam, and they've been in service for years without any problems.

 

As a designer and manufacturer of solar heat exchangers, we chose ss for the very reasons listed in other replies. Though generally superior to copper and plastics, there are some potential problems in pool use, such as high chemical shock applications or "seeding" w/tablets (where one places Cl pellets in the skimmer just prior to entry into the pump/system). If a water test sample is taken in the main pool, low Cl levels can be misleading in comparison to those taken @ entry into the exchanger. Warm water temperatures will accelerate the corrosion, especially on units not equipped w/a drainback for after the pumps shut off and no flow occurs. This process is problematic in that shocking and seeding are typically used in warm weather to deter algae growth, thus both high chemical and temperature levels coincide and compound any negative effects. Still this process takes years.

Additionally, depending on the size and expereince level of your local water providor, we have seen cases where either maintenence is performed or a pipe is broken and high levels of Cl or other chemicals are added as a response to kill potential bacteria. Though this activity is infrequent and considered negligible in their effects to ss, I have lived in a small rural Illinois town where over-adding was regularily done both because of high maintenence on a failing infrastructure and out of ignorance. The ongoing addition of high concentrations of chemicals caused both copper and ss to suffer due to cummulative effects, especially on hot water lines on small households where water sat for prolonged periods. In this case, though, you have more to fear regarding your health than the nuisance of replacing pipes.

Short answer, if you live in a major metropolitan area, I wouldn't worry.

 

[rastleman]

I am designing a system for use on city water in which a 316 SS spring contacts a 304SS retaining washer. I know that there will be galvanic action, but do not know how to predict how fast this will occur. Intuition tells me that it won't be significant because the metal nobilities are very close.

Can someone give me feedback on this question? Greatly appreciated,

rastleman

 

[EdStainless]

CSCC of 304 and 316 in hot potable water sysems is common. I have seen dozens of cases where there was no MIC or fouling that had severe cracking.
The reason that the chloride level isn't too important is that low concentrations are enough. Excess chloride doesn't speed the cracking.
Temperature is the most important factor.

Galvanic effects between 304 and 316 are rare. As long as they are both passive there will be no problem. If one of them starts to corrode then things will go quickly.
Why are you using stainless? Even though I like it, copper based alloys may be better for what you are doing.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rastleman]

Thanks, Ed Stainless. I am using SS because one of the components is a 316 SS spring that has been designed to meet spring constant in the geometry provided.

We simply did not consider copper or copper alloys before...

 

[TVP]

As EdStainless already indicated, galvanic corrosion will not be significant. SCC or crevice corrosion is definitely a concern if Cl ions and elevated temperatures will be present.