Purity of knife steel
Non-metallic inclusions, shown in picture below, are always weak points in the steel. They are starting points for corrosion and crack initiation points that reduces toughness.
Sandvik chromium knife steels have been used for decades all over the world in the food and health care industries because of their high level of purity considering non-metallic inclusions.
The yellow arrow in the picture at the left shows one example of a non-metallic inclusion that may act as a starting point for corrosion and cracks. The Sandvik steel to the right has much lower impurity content and is free from primary carbides, enabling a very high toughness and good corrosion resistance.