The appearance of defects on the surface of metals suggests low quality steel. Even more questions arise when brown rust spots appear on high alloy steel. Stainless steel is resistant to chromium as the leading alloying element. Even a small amount of chromium in the alloy helps form the thinnest chromium oxide film, which prevents corrosion due to the influence of aggressive reagents, water, and alkalis.
Effective additives that provide high-alloy alloys with anti-corrosion properties don’t always solve problems with defects. There are six main types of rust that affect stainless steel. It is worth talking about them in more detail:
- Crevice corrosion. When designing products and massive metal structures from a galvanized steel sheet, gaps may arise or the fixing points are not well sealed. Gradually water or acidic reagents will deactivate the oxide layer. If appropriate measures are not taken in time in which the reaction stops, not only the attachment point will rust but also the fasteners.
- Pitting corrosion. It occurs in violation of the technology of working with stainless steel. The aggressive external environment, small particles of metal dissolve the protective oxide layer, penetrating deep into the alloy, forming pits.
- Galvanic corrosion. The condition for its manifestation is a conductive medium. Stainless steel comes into contact with aggressive reagents and destructive properties of the protective chromium layer are fully manifested.
- Intergranular corrosion. There are conditions under which steel crystals fall out during stainless steel welding. Point gaps are formed, in which rust subsequently develops.
- General corrosion. It occurs when iodine, chlorine, fluorine gets on the surface, destroying the molecular structure of the chromium-containing protective layer.
To ensure that the stainless steel meets the technical requirements, the following manipulations are carried out:
- X-ray fluorescence analysis (XRF) is one of the most popular analytical methods for elemental analysis. Whether you are measuring liquids, powders, or solid samples, XRF combines high accuracy with simple and fast sample preparation. Optical Emission Spectroscopy can analyze elements from fractions of ppm to percentage levels from trace analysis in pure metals to complex alloys, optical emission spectrometers. All important elements are measured simultaneously. Each of these two methods has its own advantages. In the case of optical emission, it is the simplicity of the manufacture of equipment and the accuracy of determining the elements. And in X-ray analysis, it is the ability to analyze micro-objects (when installing the attachment on a scanning electron microscope), such as non-metallic inclusions.
- Thickness gauge for measuring steel. Cold rolling is a metal forming by pressure, which is carried out at a temperature below the recrystallization temperature (500º – 600º), while metal hardening is achieved. During cold rolling, the grains are deformed, stretched in length. A change in the structure leads to a change in the properties of the deformed metal: the strength and hardness increase, and the plastic properties of the metal decrease. Other properties also change, for example, the magnetic permeability drops sharply. This requires accurate elemental analysis.
- Metal analysis software. It helps laboratories analyze metal samples in steelmaking. Foundries need to select the software that best suits their production goals.