Stainless steel in potable water 4

[apsix]

Is 304 grade stainless for roof support columns acceptable in a potable water reservoir (with low chlorine dosage) or would 316 be recommended?

Thanks, John

 

[TVP]

Exactly how low is the Cl level? I doubt that 304 will be sufficient.

 

[stanier]

There are some great articles on the NIDI website

http://www.nidi.org

that cover this matter.

I also doubt that 304 would be adequate. There are even articles re a 316 structure collapsing due to stress corrosion cracking even though the temperature was ambient. You may need to consider a super duplex.

Most of the stainless steel manufacturers have a metallurgist on hand to advise.

 

[EdStainless]

316 in common in this service, though failures are not unheard of.
A lean duplex alloy would be a good option. These alloys (2003, 2101 and such) have a lower alloy content than the traditional 2205 which reduces their cost. The still have the SCC resistance of other duplex alloys. Their pitting resistance is similar to 316. And of course they still have the high strength of duplex alloys.

In todays market the lean duplex alloys will be less expensive than 316, and if the design is optimized to use their strength, maybe even less than 304.

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Corrosion never sleeps, but it can be managed.

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

 

[unclesyd]

There should be some literature on this process as it was the primary way to recover ammonia vapors from different processes at the time.

We have two million gallon potable water CS storage tanks were we had failure of the CS roofs. Both tanks were fitted with an Aluminum roofs with 304 SS center supports. Pitting, in our case associated with the bugs of rust, was discovered on one tank’s support and the supports were changed to Hastalloy C. We had some surplus pipe.

Examination of the pitting revealed extensive bug damage but no SCC. The support from the second tank was in pristine condition no sign of any attack.
The pitted pipe was 4-5 years old and other about 3.
These tanks are inspected and cleaned on a yearly (±) schedule. The inspection schedule is going to be extended to 3 yrs.
Interesting note:
According to the inspector there is no discernible corrosion where the Aluminum roof is attached to the CS tank.

 

[stanier]

Information is also available from this site. One article covers the corrosion of ss in the water industry.

http://www.bssa.org.uk/index.htm

 

[apsix]

Thanks for the responses. Our Cl level is about 4 ppm. and we will definitely make the contractor comply with the original spec. and use 316 stainless.

John

 

[bimr]

In the past the difference in cost was that 316 SS 10% more.

You should be aware that fabrication details are important. Use L grades when welding, have full penetration welds, and remove surface contamination and heat tint.

 

[EdStainless]

At todays Ni and Mo prices the premium for 316 is in the 15-20% range over 304.

The bargins in stainless these days are the lean duplex alloys. Yes, you need to redesign your entire project in order to take advantage. But when you can get similar pitting resistance, significant CSCC resistance, and double the strength for a slight cost premium, you would be foolish to not redesign. If strength/load is a factor at all, these alloys will save considerable money over 316.

These alloys are roughly 20% Cr and 2% Ni. The structure is 50/50 austenite and ferrite. All of the standard practices that work well with 2205 work fine with these grades. There is considerable experience in the pulp and paper industry in building tanks and silos using these alloys.

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

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

 

[rolledalloy]

Agree with the duplex options that Ed points out. 316L has been a standard, but I have seen 2205 duplex used when higher strength was required. If a lean duplex like LDX 2101 (21Cr-1Ni)was available back then it would have been the material of choice due to cost. At this time, a lean duplex like 2101 offers roughly a 10 to 20% price advantage versus 316L at least in flat products. Even without a redesign to take advantage of its significantly higher yield strength, LDX 2101 would be cheaper than 316L.

The lean duplex grades are currently in the stage of moving from a project type alloy to being available from service center inventories. Our LDX 2101 products will be available in late May.

http://www.rolledalloys.com

 

[bimr]

"Stainless steel grades, such as the 304 or 316 types are generally suitable for storing and handling cold or unheated drinking (town's) waters.
Localised corrosion by crevice or pitting mechanisms is not usually a hazard in properly designed, fabricated and finished tanks handling clean waters of drinking quality."

http://www.bssa.org.uk/index.htm

"Type 304 and 316 stainless steels are normally successfully used in the range of water compositions encountered during water and waste water treatment. They do not experience general thinning and therefore do not require a corrosion allowance. This allows thin and light weight sections to be made that are readily transportable, and have a low maintenance requirement throughout a long service life. They can withstand flow rates in excess of 30m/s and behave well under turbulent conditions. Aeration processes do not lead to higher corrosion rates, as might occur with carbon steels. Stainless steels do not need chemical additives to the water to maintain corrosion resistance. Metal leaching rates are minimal.

If corrosion occurs at all in waters, it is most likely to be in the form of crevice corrosion. Experience indicates that crevice corrosion is rare for types 304 and 316 when the chloride levels are below 200ppm and 1000ppm respectively. Nevertheless, it is also good practice to avoid tight crevices where possible during design, fabrication and when in service.

Between 1000 and 3600 ppm of chlorides duplex grade 1.4462 (2205) can be considered. Where conditions are exceptionally severe, 6% Mo austenitic or superduplex stainless steels should be considered. These steels can be useful in seawater applications where chloride levels can be as high as 26,000 ppm and in the presence of crevices.

Other good practices are to use L grades when welding, have full penetration welds, and remove surface contamination and heat tint. Water should be drained promptly after hydrotesting, and dead legs and other stagnant conditions should be avoided in service because flowing conditions help keep surfaces clean. Over-chlorination should be avoided as well as dosing sodium hypochlorite or ferric chloride onto the metal wall rather than into the flow stream where it can become well mixed. Areas where chlorine vapours can collect should be well vented or periodically washed down with potable water to avoid concentration of condensed chlorides."

http://www.bssa.org.uk/index.htm

I will note that the chloride ion (not chlorine) concentration in potable water is generally less than 200 mg/l. Chlorine concentrations are generally less than 2 mg/l.

I have worked in the water/wastewater industry for years and have seen extensive use of 304L with no problems.