Why does stainless steel rust？

All steel will rust, the key is determined by the material and quality of the protective oxide film. Because some steels contain more than 13% chromium, they have much higher rust resistance than ordinary steels, and they are called stainless steels at this time.
2. The reason why stainless steel “does not rust”

Iron rusts because it easily reacts with oxygen in the air to form brittle red iron oxide. Stainless steel is corrosion-resistant, that is, “no rust” because there is a layer of chromium-rich oxide film on the surface.

3. The main external factors that cause metal rust:

(1) Atmospheric relative humidity At the same temperature, the percentage of atmospheric water vapor content and its water vapor saturation content is called relative humidity. Below a certain relative humidity, the metal corrosion rate is very small, and above this relative humidity, the corrosion rate increases sharply. This relative humidity is called the critical humidity. The critical humidity of many metals is between 50% and 80%, and that of steel is about 75%. The relative humidity of the atmosphere has the greatest impact on metal corrosion. When the atmospheric humidity is higher than the critical humidity, water film or water droplets appear on the metal surface. If harmful impurities contained in the atmosphere are dissolved in the water film or water droplets, it becomes an electrolyte and aggravates corrosion.

(2) Temperature and humidity

Both atmospheric temperature and humidity affect metal corrosion. This has the following main conditions: First, the water vapor content of the atmosphere increases with the increase in temperature; second, the high temperature promotes the intensification of rust, especially in the humid environment, the higher the temperature, the faster the rust rate. When the relative humidity is low, the influence of temperature on rust is not obvious, but when the humidity is higher than the critical humidity, the amount of rust increases sharply as the temperature rises. In addition, if there is a temperature difference between the atmosphere and the metal, condensed water will form on the surface of the metal with a low temperature, which will also cause the metal to rust.
(4) Other chemical factors

The atmosphere contains a lot of dust, such as smoke, coal ash, chloride and other acid, alkali, salt particles, etc., some of which are corrosive, or condensation nuclei of water droplets, are also rust factors, such as chloride is considered It is the “mortal enemy” of corroding metals