Nine powerful strategies to combat \"valve corrosion\"”

Thread Content

Nine effective methods to combat “valve corrosion” – Reposted from the Internet (Minnan classical music). Metal valves can be considered the key components in engineering equipment that are most prone to corrosion and failure. Generally, materials of the first grade are used for valve parts such as sealing surfaces, valve stems, diaphragms, and small springs; materials of the second or third grade are used for valve bodies and covers. For valves intended for use with high-pressure, highly toxic, flammable, explosive, or radioactive media, materials with minimal corrosiveness are selected.

**Valve corrosion**
Under complex working conditions such as in the atmosphere or various solutions, metal valves not only experience uniform corrosion on their surfaces, but also suffer from localized corrosion phenomena like pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, erosive wear, cavitation corrosion, fretting corrosion, and hydrogen embrittlement at certain locations.

**Anti-corrosion measures for metal valves**
1. **Select corrosion-resistant materials based on the corrosive medium**
In practical production, the corrosive nature of media is extremely complex. Even when using the same type of valve material in the same medium, variations in concentration, temperature, and pressure lead to different degrees of corrosion. For instance, for every 10°C increase in medium temperature, the corrosion rate rises by approximately 1–3 times. Medium concentration also significantly affects corrosion; lead experiences minimal corrosion in dilute sulfuric acid, but when the concentration exceeds 96%, corrosion increases sharply. Conversely, carbon steel suffers the most severe corrosion at a sulfuric acid concentration of around 50%; above this level, corrosion decreases dramatically. Aluminum exhibits strong corrosion resistance in concentrated nitric acid (>80%), yet its corrosion rate rises in dilute or moderately concentrated nitric acid. Although stainless steel demonstrates excellent resistance to dilute nitric acid, it corrodes more severely in concentrated nitric acid (>95%).

From these examples, it’s evident that selecting appropriate valve materials requires analyzing all relevant factors affecting corrosion and referring to specialized anti-corrosion manuals.

2. **Utilize non-metallic materials**
Non-metallic materials possess excellent corrosion resistance. Provided their temperature and pressure tolerances meet requirements, they can effectively prevent corrosion while saving precious metals. Valve bodies, covers, linings, and sealing surfaces are commonly made from non-metallic materials; gaskets and packing materials are likewise produced from such materials. Plastics like polytetrafluoroethylene and chlorinated polyether, along with rubbers such as natural rubber, neoprene, and nitrile rubber, serve as valve linings, whereas valve bodies and covers remain typically cast iron or carbon steel. This combination ensures both structural strength and corrosion protection. Tube-clamp valves are designed leveraging rubber’s outstanding corrosion resistance and flexibility. Nowadays, plastics like nylon and polytetrafluoroethylene, alongside natural and synthetic rubbers, are increasingly employed to create various sealing surfaces and rings for valves; these non-metallic materials exhibit superb corrosion resistance and sealing capabilities, making them ideal for applications involving particulate-laden media. Naturally, their mechanical strength and heat resistance are relatively low, limiting their application scope. The advent of flexible graphite has enabled non-metallic materials to be utilized in high-temperature environments, resolving longstanding issues related to leakage in packing and gaskets; it also functions as an excellent high-temperature lubricant.

3. **Surface treatment of metals**
At valve joints, galvanization, chromium plating, and oxidation (bluing) treatments are frequently applied to enhance resistance against atmospheric and medium-induced corrosion. Other fasteners undergo similar surface treatments, supplemented by phosphating where applicable.

For sealing surfaces and smaller-sized closing elements, processes like nitriding and boriding are employed to improve corrosion and wear resistance. For example, valve discs fabricated from 38CrMoAlA require a nitride layer thickness of ≥0.4 mm.

To protect valve stems, surface treatments such as nitriding, boriding, chromium plating, and nickel plating are widely adopted to boost corrosion and abrasion resistance. The choice of treatment depends on stem material and operating environment: for stems exposed to atmospheric moisture and asbestos packing, hard chromium plating or gas nitriding proves effective (ion nitriding is unsuitable for stainless steel); in sulfurous atmospheres, high-phosphorus nickel electroplating provides superior protection; 38CrMoAlA benefits from both ion and gas nitriding, though hard chromium plating is discouraged. After quenching and tempering, 2Cr13 resists ammonia corrosion; similarly, carbon steel treated via gas nitriding becomes ammonia-resistant, whereas all nickel-phosphorus coatings lack such resistance. Gas-nitrided 38CrMoAlA boasts exceptional corrosion and overall performance characteristics, making it a popular choice for valve stems.

Small-diameter valve bodies and handwheels are often chrome-plated to improve corrosion resistance and aesthetic appeal.

4. **Thermal spraying**
Thermal spraying constitutes a modern technique for applying protective coatings; it utilizes high-energy-density heat sources—such as gas flames, electric arcs, plasma arcs, resistive heating, or detonation—to melt metallic or non-metallic materials, which are then atomized and sprayed onto pre-treated surfaces, forming a coating. Alternatively, simultaneous surface heating may induce re-melting of the coating, resulting in a metallurgically bonded layer. This method can apply coatings onto virtually any substrate, including metals, alloys, metal oxides, ceramics, cermets, and hard metal compounds.

Thermal spraying enhances surface properties like corrosion resistance, wear resistance, and thermal stability, thereby extending service life. Special functional coatings can provide insulation, electrical isolation, self-lubrication, thermal radiation control, or electromagnetic shielding; damaged components can also be repaired via this process.

5. **Painting and coating**
Painting remains one of the most widespread anti-corrosion strategies; for valves, it serves both protective and identification purposes. Composed primarily of synthetic resins, rubber emulsions, vegetable oils, and solvents, paint forms a barrier between metal surfaces and corrosive agents. It is particularly suited for environments with moderate corrosivity, such as water, brine, seawater, or ambient air. Interior valve surfaces are routinely painted to shield them from moisture and atmospheric effects. Different colored paints denote specific material compositions. Painting frequency typically ranges from semi-annually to annually.

6. **Addition of corrosion inhibitors**
Corrosion inhibitors function by promoting electrochemical polarization. They are chiefly applied within media and packing materials. Adding inhibitors to fluids reduces equipment and valve corrosion; for instance, chromium-nickel stainless steel tends to corrode severely in oxygen-free sulfuric acid across broad concentration ranges; however, introducing minute quantities of oxidizers like copper sulfate or nitric acid induces passivation, forming a protective film. In hydrochloric acid, small amounts of oxidizers mitigate titanium corrosion. Water is commonly used as a test medium for valve pressure testing, but it may induce corrosion; adding sodium nitrite counters this effect. Asbestos packing contains chlorides that aggressively attack valve stems; distillation-based washing can lower chloride content, though implementation challenges limit its general applicability—it’s reserved for niche scenarios.

To safeguard stems from asbestos-induced corrosion, inhibitors and sacrificial metals are incorporated into packing. Sodium nitrite and sodium chromate generate passive films on stems, enhancing corrosion resistance; solvents facilitate gradual inhibitor dissolution while providing lubrication. Zinc powder, acting as a sacrificial metal, preferentially reacts with chlorides in asbestos, minimizing direct contact with stems—effectively functioning as another inhibitor. Incorporating red lead or calcium plumbate into paints creates additional protective layers against atmospheric corrosion.

7. **Electrochemical protection**
Two primary electrochemical protection methods exist: cathodic and anodic protection. Zinc serves as a classic sacrificial anode; when used to protect iron structures, zinc corrodes preferentially. In practice, cathodic protection is far more prevalent than anodic protection. Large or critical valves frequently employ cathodic protection due to its cost-effectiveness and reliability. Embedding zinc within asbestos packing also exemplifies cathodic protection.

8. **Controlling the corrosive environment**
Environmentally, “environment” can be interpreted broadly or narrowly: broadly, it encompasses surroundings and internal media; narrowly, it refers solely to local conditions. Most environmental factors are uncontrollable, and industrial processes rarely permit modification. Only when no adverse effects on products or processes arise should environmental controls be implemented—examples include deoxygenating boiler water or adjusting pH levels in petroleum refining. From this perspective, inhibitor addition and electrochemical protection also constitute environmental management tactics.

Atmospheres contain dust, moisture, and fumes; especially in industrial settings, emissions from chimneys and machinery contribute to varying degrees of valve corrosion. Operators must adhere to protocols: regularly cleaning, purging, and lubricating valves mitigates environmental corrosion. Installing protective covers over stems, utilizing underground chambers for buried valves, and painting valve surfaces further minimize exposure to corrosive substances. Elevated temperatures and air pollution accelerate corrosion, particularly in enclosed spaces; thus, open workshops or ventilation/cooling systems are preferable to slow degradation.

9. **Optimizing manufacturing processes and valve designs**
Valve corrosion prevention must be addressed during design phases. Well-structured designs paired with optimal fabrication techniques significantly reduce corrosion risks. Design and manufacturing teams must refine components prone to corrosion due to flawed structures or inappropriate processing. Below are targeted solutions for common corrosion types:

- **Preventing intergranular corrosion in austenitic stainless steel**: Perform “solution annealing” by heating to ~1100°C followed by rapid water quenching; opt for low-carbon (<0.03%) grades enriched with titanium and niobium to suppress chromium carbide formation.
- **Mitigating stress corrosion cracking**: Eliminate residual stresses through heat treatment, revise poorly designed structures to avoid stress concentrations, and deploy electrochemical protection or anti-corrosive coatings. Inhibitor addition and compressive stress induction also help.
- **Countering erosive wear**: Select wear- and corrosion-resistant materials, refine structural designs, and employ cathodic protection.
- **Addressing fretting corrosion**: Apply lubricants to reduce friction, perform surface phosphating, utilize hard alloys, or enhance surface hardness via shot peening or cold working. Post-welding annealing is essential. Higher surface roughness correlates with improved corrosion resistance. Optimizing packing/gasket designs—using flexible graphite, plastic composites, graphite-adhered gaskets, or PTFE-wrapped variants—improves sealing efficiency and minimizes stem/flange corrosion.