“In practice the cause of corrosion is very often a consequence of final surface treatment,“ said Florian André from Henkel Pickling and Electropolishing. As an example, he mentioned that corrosion resistance behaviour of stainless steel alloys is easily disrupted by means of conventional mechanical and welding shop procedures during component production. “Such very typical surface damages can be eliminated by surface treatments such as chemical pickling or electrochemical polishing reducing the risks of later corrosion damage.“ But – probably the main challenge – damages have to be realised in the first place.
Corrosion surrounding stainless steel
In hydraulic engineering, a rise in the use of stainless steels like chrome-nickel alloys (CrNi) have increased coating and corrosion damage of adjacent structural steel parts, said Günter Binder from the Federal Waterways Engineering and Research Institute Karlsruhe.
The use of CrNi-steels on sealing and clamping strips, abrasive surfaces or alignments for hydraulic steel structures accelerates the attack on construction steel (Baustahl) in two ways:
● blistering of the construction steel coating and
● high corrosive abrasion rate of the construction steel (bimetallic corrosion). Here, the construction steel works against the "noble" CrNi steel – which forms the cathode - as the less noble anode.
Bimetallic corrosion can be avoided by selecting appropriate materials or integrating other protective measures like coating during the design state. “Since in many cases, the prevention or reduction of CrNi steel is not feasible, nor is the installation of a cathodic protection system, the coating of the nobler, cathodically acting CrNi-alloy should be considered to limit bimetallic corrosion,“ Binder said. Research on coating of CrNi-alloys showed that a coating without injury withstands cathodic current loads. Here a challenge is the quality of adhesion of the coating on such surfaces.
Be flexible: Change the material
Stainless steel 1.4571 has been a standard for many years. But when this titanium stabilised grade of stainless steel was first developed, steel-making technology was not as far advanced as it is today and carbon was difficult to remove from the molten steel. “With more recent developments the widely used 1.4571 became obsolete,” argued Frank Wilke from Deutsche Edelstahlwerke, a company focused on the production of specialty steel products. Modern processes such as vacuum oxygen decarburisation (VOD) now allows economical removal of carbon from the steel. In addition to minimising the possibility of sensitisation during welding or high temperature processing, the low carbon grades have overcome the surface problems commonly experienced with the titanium-stabilised grades.
“Many of the 1.4571 applications can now be replaced by 1.4404, a similar austenitic, highly corrosion-resistant steel. Other alternatives are ferritic steels like 1.4521 or the duplex steel 1.4362/1.4462,“ concluded Wilke.
