Pumping oxygenated seawater is just about the worst thing you can do to carbon steel. The problem is not so much pitting, as uniform corrosion because of oxygen, which is accelerated by the presence of chlorides (higher electrical conductivity) and fluid velocity.
The short answer to your question is that coating tubing is fiendishly difficult, especially at connections. One mistake or a wireline run and you get plenty of exposed metal that will be attacked. The other big issue is that unless your reservoir is extremely reducing (say, lots of iron), you will quickly find oxygen in your producing well, surface facilities and flow lines - and you will have a big corrosion problem with no easy or cheap solution.
I have seen corrosion rates of 1.8 mm/year in a few field that were reinjecting brine without deoxygenating it first, even though coated tubing had been in use for years. Use of oxygen scavenger can reduce metal loss by a factor 5 to around 0.35 mm/year.
The advantage you have is that warm water contains far less oxygen than cold water (you may have half of the oxygen concentration of the North Sea), and that salinity is lower than a reservoir brine's. Nonetheless, you will be eating through your tubing in less than 3 years.
There are other issues with injecting untreated seawater, but I'm sure you have considered them already: you have to be thorough at removing microorganisms (as well as shellfish and fish), seawater chemistry is often unsuitable for hotter rock and reservoir brine (sulphate scaling is all too common), and it has plenty of nutrients to support bacterial colonies (microbial corrosion and reservoir souring). Of course, any near-wellbore damage will likely be bypassed by thermal fracturing, but that's another story. If you're redesigning your seawater treatment, you may want to look at unwanted side effects on scaling and reservoir damage as well.
Best regards,
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Matteo Loizzo
Well integrity consultant
Berlin