Experiment by a BUT student in collaboration with FK system: possible explanations for thread dewetting

25. 8. 2021

The work included the analysis of a component where the M8 thread had become unthreaded during pickling - the aim was to find out why this happened. This component (called a showerhead in company slang), which is part of a wastewater treatment plant, was ordered for pickling by IN-EKO Team.

Figure 1: Shower unit

Several possible hypotheses have been established as to why the coil uncoiling occurred. One of them was that the pickling time on which the experiment was based was too long. A measurement of the time-dependence of the weight loss was made and a visual inspection of the surface quality after pickling was carried out in collaboration with our in-house technician.

The experiment showed that thread stripping did not occur even after an excessively long pickling time of 29 hours. The weight loss was then plotted by the student on a graph, which shows the movement over time - although there was an exponential weight loss in the later stages of pickling, there was still no degreasing even after a very excessive time.

Visual inspection of the thread surface quality showed that the optimum pickling time was 4 hours. If the pickling time was shorter, scale residues could still be discerned on the thread surface. With longer pickling times, the base material had already begun to attack.

Figure 2: Weight loss graph

So what was behind the thread derusting? No error was found in the welding process and even the extremely long pickling did not result in complete derusting. Marek Hendrych believes that the material may have been confused with another type of stainless steel during the production of the thread.

Based on his findings, he subsequently created a customer manual in which he summarizes the most common mistakes that the pickling shop's customers make. The manual could help customers avoid these mistakes in the future so that the pickling shop can derust the parts correctly.