Chromium (periodic table symbol: CR, The most common stable isotope is 52Cr.
Chromium is the main additives to most stainless steels. It can reach 25-30% by mass of stainless steel. The corrosion resistant function of chrome starts to be effective at about 10% chromium.
Chromium on Mars
Production of stainless steel on Mars would require chromium. As the cosmic abundance of chromium is fairly high, it should be available on Mars. Chromium will probably be found in conjugation with other metals, either in cooled lava chambers, asteroid impacts or in regions of hydrothermal vents.
Chromium is biologically active. The precise amount required is debatable and some forms of chromium compounds are poisonous. So some chromium is likely to be required in the food chain, but it will probably come in through the preparation of Martian regolith for soils to grow plants in.