Effect of corrosion inhibitor modified silane pretreatment on the performance of electrical steel sheets and laminates

Epoxy varnishes for stacked electrical steel are essential for further development of renewable energy and electric mobility technologies. To enhance the efficiency of the stator, direct cooling solutions are under development. Adherence of the epoxy insulation to the oxidic surface of electrical steel under service-relevant conditions is essential. Hence, silane- and corrosion inhibitor-based pretreatments were assessed as to their impact on the corrosion resistance and the mechanical performance of bonded electrical steel laminates. Low molar mass silanes with glycidyl (GS) or amine (AS) groups were used, along with inorganic and organic corrosion inhibitors, including bis-(2-ethylhexyl) phosphate, cer(III)nitrate, benzimidazole and benzotriazole. Corrosion resistance was tested under alternating humidity and temperature (AHT) and via cyclovoltammetry. Moreover, the roll peel strength (RPS) and fatigue crack growth kinetics (FCG) were examined.

Corrosion tests revealed superior performance for aminosilane pretreatment. Organic inhibitors slightly improved AHT performance. However, aminosilane pretreatment reduced the roll peel strength, whereas glycidylsilane pretreatment with inhibitors had no significant impact. Fracture mechanics testing was much more sensitive resulting in a similar ranking. Assessing the fatigue crack growth performance at elevated temperature and humidity, different failure modes were observed. The reference exhibited mixed-mode failure. In contrast, mainly interfacial failure was detected for the aminosilane pretreatment. Laminates with glycidylsilane pretreatment exhibited a mixed mode and interfacial failure at high and low propagation rates, respectively. Overall, no clear positive effect of the investigated pretreatment systems on the performance of electrical steel laminates was deduced.