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Corrosion is more than visible rust
Corrosion is damage caused by a metal’s reaction with its environment. It can cause gradual surface loss or small, deep pits. A clean-looking metal does not necessarily resist every environment equally.
Nickel alloys in chemical plants help limit these different effects. Composition changes which liquids and temperatures they can withstand. Corrosion resistance is therefore not one universal number.
What do nickel, chromium and molybdenum contribute?
Nickel contributes to the alloy structure and resistance in many chemicals. Chromium helps form a protective surface layer. Molybdenum supports resistance in selected acids and against localised chloride attack. Alloys combine these elements in different proportions.
Pure nickel’s usefulness in alkalis does not mean the highest nickel content is best for every chemical. Chromium–molybdenum alloys such as 625 and C-276 are used for the properties of the combination rather than one element alone.
What changes in salt water and tight crevices?
Chlorides in salt water can cause small, deep pits in some metals. Tight regions beneath gaskets receive less fresh liquid and develop conditions different from open surfaces. Crevice corrosion can occur there.
Resistance to these localised forms of corrosion is one reason to use alloys such as 625, C-22 and C-276. A hotter or more concentrated liquid can change their behaviour; exposure matters alongside alloy identity.
Different options in chemical equipment
825 serves acid processes and exchangers. C-22 and C-276 are widely used in reactors, evaporators and chemical lines. Each has its own balance of chromium, molybdenum and other elements.
A liquid’s trade name alone does not describe every condition. Other substances, temperature and concentration change its effects on metal. Understanding application ranges is more useful than a single best-to-worst ranking.
Hot-gas oxidation and liquid corrosion
A liquid-contact metal and a furnace metal experience different surface effects. Retention of a protective oxide matters in furnace alloys; behaviour in acid depends on different chemical conditions.
Oxidation resistance of 601 and chemical-equipment use of C-276 cannot be described by one measure. Both can resist attack in different environments, illustrating the range of the family.