These Polished Chrome door numbers and letters are made from 316 grade stainless steel, and are specifically designed for areas which are within a 25 mile radius of the coast, or within 5 miles of highly populated industrial areas. Unlike brass and zinc based products, these door knockers will not pit, rust or corrode no matter what environments you put them in. This means that the products will last longer. These front door numbers and letter have been salt spray tested to ASTM B117 for 2,000 hours, and comes with an industry first LIFETIME coating guarantee.