Nickel-Chromium High-Temperature Oxidation Layer
A dense, chromium-rich protective oxide scale ($\text{Cr}_2\text{O}_3$) formed on nickel-base alloys under high thermal exposure, acting as a diffusion barrier to protect the material from scaling.
The oxide layer on nickel-based alloys forms from the reaction of chromium with oxygen at high temperature. This creates a dense, firmly adhering layer that protects the base material from further scaling and corrosion.
When welding, you have to be careful not to destroy this layer unnecessarily. If you let the alloy get too hot or the shielding gas coverage is not clean, the chromium burns out and the layer loses its protective effect.
In practice, this means: work cleanly, keep the heat under control, and don't cool down too quickly. Then the layer remains intact and the component performs as promised.