MechChem Africa

When stainless pipework is installed, it is usually pressure tested, which is generally done by pumping water into the pipework. If the water is left in the pipe after the successful completion of the test, it creates deaerated conditions, which starve the stainless of oxygen, preventing the formation of the passive layer on the inside surfaces.

John Tarboton, executive director of Southern Africa’s stainless development association, talks about stainless piping and the need to remember that stainless steel surfaces need oxygen in order to maintain their protective passivation layer. “So the corrosion protection breaks down and, within months, pitting corrosion can destroy the pipe,” begins Tarboton.

Oxygen an essential ingredient for stainless passivationThe transparent and adherent passive film on stainless steel requires oxygen in order to self repair.

“I have seen this countless times,” he continues, “with 3CR12 stainless steel in the mining industry and, most recently, in brand new 304L pasteurising holding pipes for a local dairy,” he notes. Explaining why, he says that, “we often find that the problem is caused by microbiologically influenced corrosion (MIC). In stagnant water conditions, bacteria grow on the stainless surfaces, initiating corrosive conditions underneath the patches of bacteria.

“But in the case of the dairy, this was not the case. The water used for the pressure test had been treated with chlorine and the cleaning agent, peracetic acid, to ensure sterile conditions were maintained inside the tube. Unfortunately, in an effort to save water – the drought was on at the time – the dairy did not drain the pipe after the test,” Tarboton says.
In the dairy industry, pasteurisation is achieved by raising the temperature of the milk and holding that temperature for a predetermined length of time: for continuous-flow pasteurisation, 72°C for a minimum of 15 seconds is common, for example. “At this dairy, after heating, the milk is pumped through about 500 m of holding tube at a controlled rate so that, by the time the milk exits the tube, it will have been pasteurised,” Tarboton explains.

The new holding tube was part of an expansion project and its installation was being done before the Christmas break last year. “Pressure testing was successfully completed just before closing for the holiday and the treated water was left inside the pipe over break,” he continues.

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