How to Select Corrosion-Resistant Valves

For valves operating under corrosive medium conditions, corrosion protection is the most critical aspect of chemical equipment. Improper selection of metallic materials for chemical valves may, with a minor oversight, result in equipment damage in mild cases, or accidents and even catastrophic failures in severe cases. Relevant statistics show that approximately 60% of chemical equipment failures are caused by corrosion. Therefore, the scientific selection of materials shall be prioritized during the type selection of chemical equipment.

A common misconception regards stainless steel as an "all-purpose material" suitable for all media and environmental conditions. This belief is incorrect and highly hazardous. Below are key points for material selection for several commonly encountered chemical media:

1. Sulfuric acid medium As one of the highly corrosive media, sulfuric acid is an important industrial raw material with wide applications. The corrosion of materials varies significantly with sulfuric acid concentration and temperature. For concentrated sulfuric acid with a concentration above 80% and temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance. However, they are not suitable for fast-flowing sulfuric acid and cannot be used for pumps and valves. Conventional stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited applicability in sulfuric acid service. Consequently, pumps and valves for sulfuric acid transportation are usually made of high-silicon cast iron (difficult to cast and machine) or high-alloy stainless steel (Alloy 20). Fluoroplastics deliver good sulfuric acid resistance, and fluorine-lined pumps and valves (F46) represent a more economical option.
2. Hydrochloric acid medium Most metallic materials cannot resist hydrochloric acid corrosion (including various stainless steels). Molybdenum-containing high-silicon iron is only applicable for hydrochloric acid below 30% concentration at temperatures up to 50°C. In contrast to metals, most non-metallic materials feature excellent resistance to hydrochloric acid. Rubber-lined pumps and plastic pumps (e.g., polypropylene, fluoroplastics) are therefore the optimal choice for hydrochloric acid conveyance.
3. Nitric acid medium Most common metals suffer rapid corrosion and degradation in nitric acid. Stainless steel is the most widely used nitric acid-resistant material, showing good corrosion resistance to nitric acid of all concentrations at ambient temperature. It is worth noting that molybdenum-bearing stainless steels (e.g., 316, 316L) do not outperform conventional stainless steels (e.g., 304, 321) in nitric acid resistance, and may perform worse in some scenarios. Titanium and titanium alloys are generally adopted for high-temperature nitric acid service.
4. Acetic acid medium Acetic acid is among the most corrosive organic acids. Ordinary steel undergoes severe corrosion in acetic acid of any concentration and temperature. Stainless steel serves as an excellent acetic acid-resistant material; molybdenum-containing 316 stainless steel can also withstand high temperatures and dilute acetic acid vapor. For demanding working conditions such as high-temperature, high-concentration acetic acid or mixed corrosive media, high-alloy stainless steel or fluoroplastic pumps may be selected.
5. Alkali (Sodium Hydroxide) Carbon steel is widely used for sodium hydroxide solutions with concentration up to 30% at temperatures below 80°C. Many petrochemical plants still employ plain carbon steel under 100°C and 75% concentration. Although corrosion increases under these conditions, the material offers favorable economy. Conventional stainless steel has no obvious advantage over cast iron in caustic media. Stainless steel is not recommended if trace iron contamination in the medium is acceptable. Titanium, titanium alloys or high-alloy stainless steels are commonly used for high-temperature alkaline solutions.
6. Ammonia (Ammonium Hydroxide) Corrosion of most metals and non-metals in liquid ammonia and aqueous ammonia (ammonium hydroxide) is minimal. Copper and copper alloys shall not be used.
7. Salt water (Seawater) The corrosion rate of ordinary carbon steel in sodium chloride solution, seawater and brine is relatively low, and coating protection is generally required. Various stainless steels also have low uniform corrosion rates, yet localized corrosion may be triggered by chloride ions. 316 stainless steel is usually the preferred grade.
8. Alcohols, ketones, esters and ethers Common alcohols include methanol, ethanol, ethylene glycol, propanol, etc.; ketones include acetone, methyl ethyl ketone, etc.; esters include various methyl esters and ethyl esters; ethers include methyl ether, diethyl ether, butyl ether, etc. These substances are basically non-corrosive and compatible with most common materials. Reasonable material selection shall be made according to medium properties and relevant requirements. Special attention shall be paid to the solubility of ketones, esters and ethers towards many types of rubber to avoid mistakes in sealing material selection.

Many other media cannot be covered here. In summary, material selection must never be arbitrary or blind. Relevant literature shall be consulted or proven field experience referenced.