Corrosion measurements save dairy manufacturers millions
Frequently, dairy companies receive faulty stainless steel pipes from various sub-suppliers. It may be all from visible pits and pores to poorly executed weldings, and at the dairy giant Arla Foods, Chief Engineer Erik-Ole Jensen works to detect such faults and their sources. However, a large part of the faults are not immediately visible and calls for laboratory tests to be diagnosed correctly. For a long period of time, Erik-Ole Jensen has been cooperating with IPU Technology Development, which carries out corrosion analyses and corrosion tests that may also identify invisible faults.
Fault detection prevents dairy company shutdown
"Arla Foods has multiple sub-suppliers of stainless steel dairy pipes, and the picture is unambiguous: In recent years, quality problems of supplied dairy pipes have increased. Last year's tests showed corrosion and faults in many supplies", Erik-Ole Jensen explains, and adds: "Losses in the triple-digit million range are at stake if a dairy company needs to close for a couple of months due to faulty dairy pipes. Typically, several kilometers of pipes from the same supplier have been used in a large dairy company, and if the faults are systematical, all pipes have to be replaced. At worst the dairy company will have to be shut down for a period of time. Arla Foods cannot live with such insecurity, which is why I spend this much time on checking the supplies."
Visible corrosion is, of course, detected at once, but there is a range of problems that are invisible to the naked eye. Erik-Ole Jensen uses IPU to a large extent to perform the detailed analysis. "Arla Foods benefits a lot from the cooperation with IPU. When, for instance, I detected an outer and inner discoloration in a supply of pipes, I began to suspect that there might be larger problems, and then asked for a test. And in this case, IPU's tests sure enough revealed corrosion problems. In another case, the problems were related to the longitudinal welded joint of the pipe. There were microscopic pores in the welding, and possibly such pores may in the course of a few weeks lead to both microbacterial growth and crevice corrosion."
Bad welding joints and deriving development of corrosion is a problem to end-users like Arla Foods, but also the small companies that are sub-suppliers are heavily affected if faults are discovered. The companies cannot themselves go through with thorough tests of the supplied dairy pipes, and will therefore have to forward the pipes to the end-users trusting the supply to be alright. Naturally, Arla Foods is keen to avoid action for damages due to misdeliveries, which is also why Erik-Ole Jensen examines the pipes thoroughly before they are mounted in the dairy company.
Improved tests and a welding catalogue
E.g. a message from Erik-Ole Jensen to IPU may be: Suspicion of problems with longitudinal welded joint or suspicion of surface with bad corrosion properties. Test for stains/pits and corrosion properties. It may also be a question of having supplies from various pipe manufacturers tested against each other. In a laboratory at the Technical University of Denmark (DTU), IPU carries out the ordered measurements, and the measured properties are forwarded in the form of a report.
Often, at IPU, the electrochemical measurements are carried out by means of a special micro electrochemical measurement equipment developed by Assistant Professor Rajan Ambat from DTU, which makes it possible to selectively examine a small area of the surface. We are not dealing with a standardised test like e.g. an ASTM test, but according to MSc Engineering Mikkel Østergaard Hansen from IPU this equipment has a number of advantages. "For instance it is easy for us to prepare the test samples, and therefore, often, the test result may be delivered after just few days. Furthermore, the sensitivity of IPU's test is higher than the sensitivity of standardised tests. We only measure small areas of the item, from 100 μm2 to 1 mm2, and by comparison ASTM tests only show average measurements over larger areas. Hereby, small deviations risk being overlooked in ASTM tests. Moreover, the technique has a lateral decomposition down to approx 200 μm, which allows us to scan and measure variations in the corrosion characteristics across the surface of an item. It could e.g. be variations in the corrosion characteristics across a welded joint.
The results from our measurements on the forwarded samples are astonishing. There may very well be a factor of several thousands from best to worst when we do ranked tests of faults in welded joints in products that are identical in principal", Mikkel Østergaard Hansen concludes.
Finally, according to Erik-Ole Jensen, often there is a lack of expertise concerning corrosion damages in stainless steel as a result of faults in connection with welding. Therefore he has asked IPU to also deliver data for a planned catalogue of the most commonly occurring welding faults and the impact on the corrosion properties. The catalogue is to contain descriptions of the full range of fault possibilities along with related photos and their consequences for the corrosion properties. In the future, in this way it will be easy to demonstrate what problems may come with the various sources of faults. Erik-Ole Jensen in this way hopes to contribute to improving the quality of both future dairy pipes and other products made from stainless steel.
Photo 1: A welding fault being examined in IPU’s test. In this case, the concentration of oxygen during the welding has been too high, and the result is increased vulnerability to corrosion (the dark traces above and below the welding joint). Photo: Carsten Broder Hansen/ScienceComm.
Photo 2: Mikkel Østergaard standing behind the newly developed test equipment in one of DTU's laboratories. The testing method has been developed by Assistant Professor Rajan Ambat from DTU, and the equipment has been built by DME - Danish Micro Engineering A/S. Photo: Carsten Broder Hansen/ScienceComm.
Photo 3: The tip of the test equipment is guided across the dairy pipe which is being examined. IPU can detect welding faults in areas with a diameter of down to 100 μm.
Photo: Carsten Broder Hansen/ScienceComm.