Zander Daniela, Schupp Alexander, Beyss Oliver, Rommes Bob, Klink Andreas
Chair of Corrosion and Corrosion Protection, Foundry Institute, RWTH Aachen University, Intzestr. 5, 52072 Aachen, Germany.
Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany.
Materials (Basel). 2021 Jan 15;14(2):402. doi: 10.3390/ma14020402.
The efficiency of material removal by electrochemical machining (ECM) and rim zone modifications is highly dependent on material composition, the chemical surface condition at the break through potential, the electrolyte, the machining parameters and the resulting current densities and local current density distribution at the surfaces. The ECM process is mechanistically determined by transpassive anodic metal dissolution and layer formation at high voltages and specific electrolytic compositions. The mechanisms of transpassive anodic metal dissolution and oxide formation are not fully understood yet for steels such as 42CrMo4. Therefore, martensitic 42CrMo4 was subjected to ECM in sodium nitrate solution with two different current densities and compared to the native oxide of ground 42CrMo4. The material removal rate as well as anodic dissolution and transpassive oxide formation were investigated by mass spectroscopic analysis (ICP-MS) and (angle-resolved) X-ray photoelectron spectroscopy ((AR)XPS) after ECM. The results revealed the formation of a FeO mixed oxide and a change of the oxidation state for iron, chromium and molybdenum, e.g., 25% Fe (II) was present in the oxide at 20.6 A/cm and was substituted by Fe (III) at 34.0 A/cm to an amount of 10% Fe (II). Furthermore, ECM processing of 42CrMo4 in sodium nitrate solution was strongly determined by a stationary process with two parallel running steps: 1. Transpassive FeO mixed oxide formation/repassivation; as well as 2. dissolution of the transpassive oxide at the metal surface.
电化学加工（ECM）去除材料的效率以及边缘区域改性高度依赖于材料成分、击穿电位下的化学表面状况、电解液、加工参数以及表面产生的电流密度和局部电流密度分布。ECM过程在机理上由高电压和特定电解液成分下的过钝化阳极金属溶解和氧化层形成所决定。对于42CrMo4等钢种，过钝化阳极金属溶解和氧化物形成的机理尚未完全明晰。因此，对马氏体42CrMo4在两种不同电流密度的硝酸钠溶液中进行ECM加工，并与磨削后的42CrMo4的原生氧化物进行比较。通过ECM后的质谱分析（ICP-MS）和（角分辨）X射线光电子能谱（(AR)XPS）研究了材料去除率以及阳极溶解和过钝化氧化物形成情况。结果表明形成了FeO混合氧化物，铁、铬和钼的氧化态发生了变化，例如，在20.6 A/cm²时氧化物中存在25%的Fe(II)，在34.0 A/cm²时被Fe(III)取代，Fe(II)含量为10%。此外，42CrMo4在硝酸钠溶液中的ECM加工强烈取决于一个具有两个并行步骤的稳态过程：1. 过钝化FeO混合氧化物形成/再钝化；以及2. 过钝化氧化物在金属表面的溶解。
