Corrosion of Admiralty Brass and Ti in Seawater 3

[Metalmeister]

A senior exec has asked me this question and I don't want to go back with an answer without checking first with water chemistry and corrosion experts on this site.

At what concentration level of sulfate (SO4) in seawater will I have a corrosion issue in admiralty brass or titanium tubed condenser boxes. I know that the pH of seawater is about 7.8 and the normal concentration of SO4 in seawater is about 0.265 wt. %. Issue relates to a seawater scrubber to remove sulfur from flue gas thereby increasing the concentration of SO4 in discharge water and potentially in surrounding water.

Are there any environmental issues related to sulfate discharge in water? Will this reduce the pH of the discharge water?

 

[TVP]

The following information is from Titanium, The Choice published by the Titanium Development Association, 1990:

1. Other salt solutions:

Titanium alloys exhibit excellent resistance to practically all salt solutions over a wide range of pH and temperatures. Good performance can be expected in sulfates, sulfites, borates, phosphates, cyanides, carbonates, and bicarbonates. Similar results can be expected with oxidizing anionic salts such as nitrates, molybdates, chromates, permanganantes, and vanadates; also with oxidizing cationic salts, including ferric, cupric, and nickelous compounds.

2. Corrosion rate of CP Ti in:
ambient seawater = nil
boiling 10-20% sodium sulfate = nil
sulfuric acid saturated with Cl (62% at 16C) = 0.002 mm/year
sulfuric acid saturated with Cl (5% at 190C) = < 0.025 mm/year

 

[Metalmeister]

Thank you, TVP, for your informative response. It is very helpful.

 

[EdStainless]

Both Ti and high alloy stainless grades (SEA-CURE and AL-6XN)have been used in seawater cooled condenser service for thirty years with very few issues.
The limits for brass are not well defined because many factors contribute to the corrosion.
The attack does not have to do with pH but with reactions that de-stabilize the passive surface films making them soluble. I have seen this in fresh water with agricultural waste in it.

Keep in mind that brass has a rather low limit on flow velocity. Both SS and Ti would like a much higher flow since it will keep the tubes cleaner. There are no erosion risks with these alloys.

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Plymouth Tube

 

[Metalmeister]

Thank you, E/S, for your tech support. Between you and TVP, I was able to give the Exec VP a good answer with a high degree of confidence.