Spikeyi
Chemical
- Apr 5, 2011
- 2
Hi,
I have a question about a 316SS foot valve in alakline brackish water, and was also hoping this post could be a bit of a reference point for high pH/ high chloride pitting questions. There seems to be a real shortage of specific information on pitting at high pH online, although high pH is generally regarded as good where SCC is not an issue.
My situation:
I'm looking at selecting a material for a foot valve for the feed pump suction line from a pond, for a reverse osmosis desalination plant in Australia.
Water has 2000 ppm chloride, pH 8.5 - 9.5, very few other dissolved species other than sodium and carbonates/bicarbonates. Turbidity = 4 NTU and not much organics content.
Low pressure as it is a suction line.
Temperature of fluid up to 35 deg C (Australia gets...warmish).
No information on dissolved oxygen.
So I'm mainly worried about pitting rather than general corrosion or SCC.
I have looked around online but most information on high pH/chloride pitting temperatures is vague.
My question:
1) What do you think of 316(L) for this application?
I currently think no but this is excluding the effect of high pH as I cant quantify it so have excluded it.
2) If not, is there a workaround like attaching some sacrificial carbon steel/some other cheap + practical fix?
And more generally:
3) What is the relationship between pH/chloride for pitting temperatures of austenitic stainless grades from pH 7 - 10? Or if this is too broad, how about just 304 and 316?
Here are two explanations I've seen at this forum, they have the same trend but quite different equivalent changes - I'm guessing they refer to different materials/pH ranges.
"
Pitting resistance in stainless steel is controlled mostly by the alloy composition as stated in FAQ 1b. The key environmental variables which influence the severity of an environment's pitting propensity are pH, chloride concentration, and temperature. Their effects are approximately additive and linear.
As a rule of thumb:
increasing temperature 25C is equivalent to lowering pH by 5 or increaing chloride concentration by a factor of ten. The effect of any of these would be offset by an increase in the PREN of 5, which in turn would represent an increase in pitting potential of 250 mV.
In addition, but less quantifiable, microbiological organisms can be harmful, increasing flow rates is beneficial,and adding inhibiting cations is beneficial." McGuire
"
There are general relationships for the tradeoff between temp, Cl and pH in stainless grades.For 304 the drop in pH of 1 is roughly equivalent to raising the Cl by 10x.In 316 you have to drop the pH by 2 to see the same impact on pitting corrosion.In other words, if 304 works at a specific temperature, Cl level and pH; then you could use 316 at the same temp, Cl level and a pH that is roughly 1 to 1.5 lower.This is only a rough estimate. You need to know specifics before you can really talk about limits. " EdStainless
Any help with this would be greatly appreciated.
I have a question about a 316SS foot valve in alakline brackish water, and was also hoping this post could be a bit of a reference point for high pH/ high chloride pitting questions. There seems to be a real shortage of specific information on pitting at high pH online, although high pH is generally regarded as good where SCC is not an issue.
My situation:
I'm looking at selecting a material for a foot valve for the feed pump suction line from a pond, for a reverse osmosis desalination plant in Australia.
Water has 2000 ppm chloride, pH 8.5 - 9.5, very few other dissolved species other than sodium and carbonates/bicarbonates. Turbidity = 4 NTU and not much organics content.
Low pressure as it is a suction line.
Temperature of fluid up to 35 deg C (Australia gets...warmish).
No information on dissolved oxygen.
So I'm mainly worried about pitting rather than general corrosion or SCC.
I have looked around online but most information on high pH/chloride pitting temperatures is vague.
My question:
1) What do you think of 316(L) for this application?
I currently think no but this is excluding the effect of high pH as I cant quantify it so have excluded it.
2) If not, is there a workaround like attaching some sacrificial carbon steel/some other cheap + practical fix?
And more generally:
3) What is the relationship between pH/chloride for pitting temperatures of austenitic stainless grades from pH 7 - 10? Or if this is too broad, how about just 304 and 316?
Here are two explanations I've seen at this forum, they have the same trend but quite different equivalent changes - I'm guessing they refer to different materials/pH ranges.
"
Pitting resistance in stainless steel is controlled mostly by the alloy composition as stated in FAQ 1b. The key environmental variables which influence the severity of an environment's pitting propensity are pH, chloride concentration, and temperature. Their effects are approximately additive and linear.
As a rule of thumb:
increasing temperature 25C is equivalent to lowering pH by 5 or increaing chloride concentration by a factor of ten. The effect of any of these would be offset by an increase in the PREN of 5, which in turn would represent an increase in pitting potential of 250 mV.
In addition, but less quantifiable, microbiological organisms can be harmful, increasing flow rates is beneficial,and adding inhibiting cations is beneficial." McGuire
"
There are general relationships for the tradeoff between temp, Cl and pH in stainless grades.For 304 the drop in pH of 1 is roughly equivalent to raising the Cl by 10x.In 316 you have to drop the pH by 2 to see the same impact on pitting corrosion.In other words, if 304 works at a specific temperature, Cl level and pH; then you could use 316 at the same temp, Cl level and a pH that is roughly 1 to 1.5 lower.This is only a rough estimate. You need to know specifics before you can really talk about limits. " EdStainless
Any help with this would be greatly appreciated.
