Oxide Formation during Transpassive Material Removal of Martensitic 42CrMo4 Steel by Electrochemical Machining.

Daniela Zander,Bob Rommes,Alexander Schupp,Andreas Klink,Oliver Beyss

doi:10.3390/ma14020402

Abstract

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 Fe3−xO4 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/cm2 and was substituted by Fe (III) at 34.0 A/cm2 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 Fe3−xO4 mixed oxide formation/repassivation; as well as 2. dissolution of the transpassive oxide at the metal surface.

Highlights

Electrochemical machining (ECM) of metals is increasingly gaining in importance in the field of subtractive manufacturing technology
Material removal of 42CrMo4 in sodium nitrate solution by ECM is determined by Material removal of 42CrMo4 in sodium nitrate solution by ECM is determined by metal dissolution, and the formation of an oxide layer and a polishing film at the surface
It is well known that the current density significantly influences the material reFurther, it is well known that the current density significantly influences the material moval rate during ECM, and the anodic dissolution and oxide layer formation, removal rate during ECM, and the anodic dissolution and oxide layer formation, at high transpassive potentials

Summary

Introduction

Electrochemical machining (ECM) of metals is increasingly gaining in importance in the field of subtractive manufacturing technology. In addition to the advantages mentioned, the ECM process shows disadvantages It is not suitable for machining specific complex geometries. The ECM process is mechanistically determined by transpassive anodic metal dissolution and layer formation at high voltages and specific electrolytic compositions. Various mechanisms are assumed to explain the anodic material removal accompanied by a transpassive layer formation of iron and steels during the ECM process. The latter is strongly dependent on the 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

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