2 replies to this topic

[Lisaer Carlo]

Hi fellow industry professionals!

My name is Carlo and am working in an oil refinery. I was tasked to determine the corrosion mechanism happening in one of our heat exchangers that was found to suffer tube thinning and pitting when we had our recent turnaround.

The specs of the said heat exchanger are as follows:
Tube Side: Cooling water, 110 m3/hr, enters at 30degC, leaves at 35 degC
Shell Side: Sour water, 33.5 m3/hr, enters at 50degC, leaves at 35 degC

During our last turnaround, our RFET test on that H.Ex. showed severe wall thinning with at least 30% metal loss on 12% of the tubes inspected (Note: There are 330 tubes on the H.Ex). Pitting was observed on the tube external, which follows that corrosion must be happening on the shell side. Due to unavailability of new tube bundle during that time, what we did was to plug those leaking tubes just so we could proceed with the startup.

On its first two months of operation, we had taken laboratory samples on the tube side inlet/ outlet and on the shell side inlet/ outlet streams. The tube side inlet and outlet did not indicate significant change in composition. Thus, migration from shell side to tube side is inexistent since leaking tubes were plugged already. Before our turnaround, migration may had occurred though we had no data to support that.

My task is to identify the corrosion mechanism in the shell side of the H.Ex. The shell side stream has a sour water quality with the following parameters (average values in 2 months of sampling).
Shell side Inlet stream: pH = 7.01, Chlorides = 864.8 mg/L, H2S = 677.2 ppm, NH3 = 763.9 mg/L
Shell side Outlet stream: pH = 7.02, Chlorides = 802.9 mg/L, H2S = 683.6 ppm, NH3 = 825.2 mg/L

As we know, H2S is corrosive and may have caused the thinning by the reaction of Fe and bisulfide ion from H2S to produce iron sulfide and iron disulfide which are both insoluble in water. For the chlorides, I would like to seek your advice what may be the mechanism of corrosion by chlorides.
1. Ammonium chloride cannot be the cause since this compound is highly soluble in water. In addition, the chlorides is around 860 mg/L only per lab result. If all these chlorides were present as ammonium chloride, that concentration is not enough to precipitate NH4Cl since solubility of this compound is 522 grams/L at the operating temp of 50degC.
2. Hydrogen chloride cannot be the cause also since HCl corrosion occurs only at an acidic pH. The pH of our sampling is at neutral at 7.

I would like to ask if chloride ions in the sour water can be the cause of the corrosion. If yes, how? If not, then what chloride compound may have been the cause at these specified conditions?

[Fr3dd]

I don't have too much knowledge about corrosion, but I have worked with sour water facilities design in the past. I guess it is some kind of an water cooler in a Sour Water Stripping plant, downstream the Stripping columns.

If that is the case, during the design phase I'm sure some previsions were made regarding the tube material. Taking this into account, if your exchanger was designed to handle sour water / cooling water I'm sure tubes must be of some metal alloy (Stainless steel maybe?). If they are SS tubes, I guess their worst enemy are the chlorides. You observe pitting, you have +800mg/l (more than 200mg/l, which could be sort of a "safe" level) of chlorine content and warm temperatures, which is concurrent with chlorine corrosion.

Maybe the chlorine concentration or the temperature is higher than expected, or the fluid velocity is too slow and is allowing the formation of deposits.

[MrShorty]

You don't list oxygen content, but I think O2 is a consideration in the corrosivity of seawater applications. Reportedly, that is why metals corrode so readily in oceanfront locations (seawater has high O2 content) where, in the deep sea (low O2), shipwrecks are preserved because the corrosivity is low.