Hot-Spot Corrosion

It is fairly well known that copper alloy heat exchanger tubes can suffer hot-spot corrosion in seawater. This usually occurs when seawater flows are low, < 0.5m/s, and process side temperatures are high. This typically occurs in clean seawater when local tube wall temperatures are ~130°C. When the seawater is polluted with sulphides, even at low concentrations, (<1mg/L), the threshold temperature decreases to 75 to 100°C.

What is less well known is that small amounts of ammonia in the seawater (1 to 2mg/L) can also cause hot spot attack. The ammonia is present as a pollutant and the seawater flow in the heat exchanger tubes does not need to be that low. Failures have been seen in heat exchangers with a nominal flow rate of 2m/s. In addition, the threshold temperature is very low and attack has been seen in tubes with very small heat transfer rates. The attack takes the appearance of rather broad pitting, which is deep in places. The pits are filled with copper oxide and there is metallic, re-deposited copper either at the base of the pit or mixed in with the copper oxide.

The alloy most susceptible to this form of attack is 90/10 copper-nickel, while the alloy most resistant (but not immune) is aluminium brass. The photo below shows pitting of 90/10 copper-nickel in the as-received condition and after acid cleaning. Once the tubes form a protective film in clean seawater, it is much harder to initiate hot-spot attack. Ferrous sulphate dosing at start-up can assist in the formation of protective films.

Acid Cleaned

                             As-Received                                                  Acid Cleaned
 

Posted on: 21st Sept 2016

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Image (top left) by Agnieszka