How Are Sanitary Valves Corroded? – 6 Forms of Corrosion and Maintenance Methods

Valve corrosion is one of the main causes of valve failure. It can lead to leaks, contamination, and even safety incidents. Sanitary valves have higher requirements for corrosion resistance.

Why Do Valves Corrode?

Corrosion is the process by which metals gradually return to their ore state through natural processes. A key electrochemical reaction is involved here: metal atoms lose electrons and turn into ions. Therefore, the key to corrosion resistance lies in whether a dense and stable protective film can form on the metal surface. In practical applications, physical effects and chemical corrosion often combine to cause valve failure.

6 Forms of Corrosion and Maintenance Methods

Sanitary valve corrosion can generally be divided into six forms. Among them, pitting corrosion is the most common and dangerous.

1. Pitting Corrosion

Pitting corrosion is a localized, deep-hole-like corrosion that forms on metal surfaces after the protective film is locally damaged, under the action of aggressive media such as chloride ions. Specifically, for sanitary valves, surfaces in contact with chlorine-containing disinfectants and areas where polished surfaces are damaged due to mechanical injury are prone to pitting.

To control pitting, the focus is on controlling the medium and environment. First, the concentration of chloride ions in the medium should be strictly limited. Second, in material selection, stainless steel with higher molybdenum content, such as 316L, can be used to enhance pitting resistance. Also, extra care must be taken during installation and maintenance to avoid any mechanical damage to the valve’s inner wall. Regular surface polishing maintenance is also necessary to preserve its integrity.

2. Crevice Corrosion

This is almost unavoidable.

Crevice corrosion occurs inside narrow gaps. Due to restricted oxygen diffusion, an oxygen concentration cell forms, leading to localized accelerated corrosion of the metal within the crevice. In particular, gasket interfaces, threaded connections, and contact surfaces between the valve seat and body on sanitary valves are high-risk areas.

Approaches to prevent crevice corrosion must be taken into consideration both in design and maintenance. For design optimization, the structure should be such as to evade the formation of harmful narrow gaps. Where joints cannot be avoided, fillings can be done using flexible gaskets or special sealants. Maintenance-wise, valves shall be taken apart on a periodic basis for inspection of possible crevice areas by careful cleaning to avoid accumulation of corrosive substances.

3. Galvanic Corrosion

Galvanic corrosion occurs when there is contact between two different metals in a corrosive electrolyte, with the formation of a galvanic cell and the ensuing accelerated dissolution of the anode metal of the lower potential. In particular, mismatched materials of the valve stem and body, mixed use of stainless steel and carbon steel bolts, or the conditions around welded versus non-welded zones are all potential risk points in sanitary valves.

Prevention is the most essential mode of protection from galvanic corrosion. Combinations of the same or electrochemically similar metals should be employed wherever possible. Where different metals must be in contact, effective isolation by non-metallic gaskets or insulating coatings can be used. Regular inspections should check valve connections for unusual rust or electrolyte deposits since these are often early indications of corrosion.

4. Fretting Corrosion and Stress Corrosion

Fretting corrosion can be attributed to the mechanical wear that causes damage to the protective film on the metal surface, and this accelerates the chemical corrosion process. It is common to find such corrosion in areas with reciprocating motion; this includes the area between the valve stem and the packing. On the other hand, stress corrosion cracking is a brittle crack caused by the combined action of tensile stress and specific corrosive media, and it is extremely hazardous.

In the case of fretting corrosion, the valve stems with hardened surfaces or wear-resistant coatings can be selected, and appropriate tightness should be given during the installation of the packing. For stress corrosion, the key is to eliminate unnecessary residual stress. Therefore, alignment by force or generating additional stress during installation should be avoided. When necessary, stress relief annealing should be performed on critical pressure-bearing components.

5. Microbial Corrosion

Microbial corrosion is localized corrosion driven or accelerated by microbial activity, essentially falling under the category of electrochemical corrosion. In sanitary valves, microorganisms tend to attach to low-flow areas, dead corners, or sealing surfaces, forming biofilms. This film alters the local environment—either consuming oxygen to form concentration cells or metabolizing to produce acidic substances, sulfides, etc.—damaging the stainless steel passive film and triggering sustained anodic dissolution.

Thus, sanitary valve systems in contact with organic-rich fluids (e.g., culture media, sugar solutions) or with stagnant flow are at high risk. The corrosion morphology often appears as irregular ulcer-like pits, accompanied by biological fouling and odors, developing rapidly and covertly.

Prevention and control depend on system design and maintenance. First, optimize pipeline and valve layout to eliminate dead corners and stagnant sections, ensuring the system can be fully drained and dried. Second, regular and effective cleaning and sterilization procedures must be strictly implemented. Valves with higher surface smoothness (e.g., electropolished) can reduce microbial adhesion. For existing biofilms, specialized cleaning agents or mechanical methods are required for thorough removal.

6. Intergranular Corrosion

Intergranular corrosion occurs along the grain boundaries of metals. In austenitic stainless steels, prolonged exposure in a specific sensitization temperature range leads to the precipitation of chromium carbide at the grain boundary, creating chromium-depleted zones that lose corrosion resistance. Therefore, weld heat-affected zones in sanitary valves and components frequently exposed to high-temperature disinfection face this risk.

To prevent intergranular corrosion, material selection is fundamental. Ultra-low carbon stainless steels, such as 304L or 316L, should be prioritized. At the same time, heat treatment processes after manufacturing and repair are crucial, as proper solution treatment can effectively eliminate sensitization effects.