The influence of delta ferrite on corrosion resistance

Delta ferrite (δ-ferrite) normally occurs in steels as a high-temperature phase above approximately 1,000 °C. In certain material groups – such as austenitic stainless steels including 1.4301, 1.4401 and 1.4571 – however, δ-ferrite can also remain stable at room temperature. In practice, the question therefore often arises as to whether this phase can adversely affect the material’s properties.

Small amounts are usually unproblematic

Moderate amounts of δ-ferrite in austenitic steels are generally unproblematic. Even after solution annealing followed by quenching, a small proportion often cannot be completely avoided. Such small amounts usually have no significant effect on the material’s corrosion resistance.

The situation is different when the δ-ferrite content is excessive. In the case shown in Fig. 2, a significantly elevated content caused existing corrosion to develop an unusual morphology and progress rapidly. The reason is that δ-ferrite has lower corrosion resistance than the surrounding austenitic microstructure. This phase was therefore preferentially dissolved.

An example of selective corrosion

The result is a striking example of selective corrosion, as shown in Fig. 3 and Fig. 4. The preferential dissolution of the δ-ferrite makes the remaining austenite plates appear to “float” freely in space. Microstructures of this kind illustrate how local differences in the corrosion resistance of individual phases can shape a material’s damage appearance.