Electrochemistry corrodes metals in different forms. It does not only function between two kinds of metals, but also can cause potential difference due to differential solubility of solution, differential solubility of oxygen and the tiny difference in internals of metals, enhancing corrosion. Some metals have poor resistance. However, they can form sensational protective film – passive film after being corroded, preventing be corroded by media further. So, to realize the corrosion resistance on
metals valves: one, eliminate electrochemical corrosion; two, if electrochemical corrosion cannot be corroded, passive film should be formed on the surface of metal; three, use non-metallic materials without electrochemical corrosion to take place of metal materials.
The followings are several corrupt prevention measures used for metal valves.
1. Choosing corrosion resistant materials based on media
In actual production, the corrosion of media is quite complicated. Even the same materials are used in the same media. Different concentration, temperature and pressure of media, the corrosion of media on materials are different. The temperature of media rising by every per 10℃, corrosion rate is enhanced about 1~3 times. The concentration of media has great influences on corrosion on valve materials. For example, when lead in sulfuric acid having low concentration, corrosion is weak. When the concentration exceeds 96%, corrosion is enhanced sharply. On the contrary, carbon steel is corroded the most seriously when the concentration of sulfuric acid is about 50%. When concentration is increased to more than 60%, corrosion is weakened sharply. Aluminum has high corrosion resistance to aqua fortis whose concentration higher than 80% while it is corroded seriously in nitric acid having medium or low concentration. Moreover, stainless steel has strong corrosion resistance to dilute nitric acid. When it is in aqua fortis whose concentration is higher than 95%, the corrosion is enhanced.
As we can see that choosing materials should be based on specific circumstances and different factors having influence on corrosion.
2. Using non-metallic materials
Non-metallic materials have good corrosion resistance. As long as working temperature and pressure of valves meeting the requirements of non-metallic materials, it cannot only figure out corrosion, but also can save precision metals. Body, bonnet, liner and sealing face of valves are usually made of non-metallic materials, including gasket and packing. Use PTFE, PCMO and other plastics, or natural rubber, chloroprene rubber, nitrile rubber buna and other rubbers to make of liner. However, the body and bonnet are mainly made of cast iron or carbon steel, which can ensure the strength and corrosion resistance. Pinch valve is also designed based on good corrosion resistance and performances of rubbers. Now, more and more sealing faces or sealing rings are made of nylon, PTFE, natural rubbers and synthetic rubbers. Those non-metallic materials used for sealing face not only have good corrosion resistance, but also have good sealing performance, particularly suitable for using in media containing particles. But, they have poor strength and heat resistance. The application is limited. The appearance of flexible graphite makes non-metallic materials be used under high temperature, which figures out the leakage on packing and gasket which is difficult to be solved for a long time. Besides, flexible graphite is also a good high temperature lubricant.
3. Spraying coating
Coating is one of the most commonly used measures for corrupt prevention, also one of essential corrosion resistance materials and identification marks. Coating also belongs to non-metallic materials, usually making up of synthetic resin, rubber slurry, vegetable oil and solvent, covering the surface of metal. It isolates media and atmosphere, to realize corrosion resistance. The coating is mainly used for environment having poor corrosion such as water, brine, seawater and atmosphere. Antiseptic paint is painted in inner chamber of valves, preventing water, air and other media from corroding valves. The paints are also mixed with different color, standing for the materials sued for valves. In general, valves are sprayed with paints once half a year or a year.
4. Adding corrosion inhibitor
Adding corrosive media or corrosives with a small quantity of other special substances can slow down the corrosion rate on metal significantly. Those special substances are called as corrosion inhibitor.
The principle of corrosion inhibitor controlling corrosion is that it accelerates polarization of battery. The inhibitor is mainly used for media and packing. Adding corrosion inhibitor into media can slow down the corrosion on equipment and valves. For example, chromium nickel stainless steel is incinerated and corroded seriously when it is in sulfuric acid which does not contain oxygen while having large range of solubility. Adding a few copper sulfates, nitric acid or other oxidizing agents can transfer stainless steel into passivity and form a layer of protective film on the surface, preventing etching of media. Adding a few oxidizing agents into hydrochloric acid can reduce corrosion on titanium. When the water which commonly used for valve pressure test is used for pressure test, it can easily cause corrosion on valves. Adding sodium nitrite into water can prevent valve from being corroded by water. Asbestos packing contains chloride having strong corrosion on stem. If asbestos packing is washed by distilled water, the content of chloride can be decreased. However, the method is difficult to be used and cannot be promoted. It is only suitable for special demands.
5. Using surface treatments
Corrupt prevention on stem also attracts attentions. People have already accumulated abundant production experience. Surface treatments including nitrizing, boriding, chromeplating and nickel plating are commonly used for enhancing corrosion resistance and abrasion resistance of valves. Different surface treatments are suitable for different materials stem and working environments. When stem is in the media of atmosphere or vapor or contacts with asbestos packing, hard chromium plating and gas nitriding (ionic nitriding not suitable for stainless steel) are used; the stem in hydrogen sulfide is electroplated by high phosphorus nickel plating, having good protection performance; ion and gas nitriding used for 38CrMoAlA also can improve corrosion resistance. However, hard chromium plating is unsuitable; 2Cr13 can resist to corrosion of ammonia after being tempered. Carbon steel nitrided by gas also can resist corrosion of ammonia while all of nickel phosphorus cannot resist to corrosion of ammonia. 38CrMoAlA nitrided by gas has good corrosion resistance and overall performances, mainly used for stem.
6. Thermal spraying
Thermal spraying is a technique for manufacturing coating, having already become one of new technologies for surface protection of materials and national key promotion projects. It is a surface strengthening method that uses high density heat source (gas combustion flame, arc, plasma arc, electric heating and gas explosion) to heat or melt metals or non-metallic materials, spraying to base surface having bee pretreated in the form of atomization and forming spraying coating, or heat base surface, melting coating again on base surface. Most of metals or their alloys, metallic oxide ceramics, metallic ceramic composites and hard metallic compounds can use one or several thermal spraying methods, forming coating on metallic or non-metallic matrix.
Thermal spraying can improve corrosion resistance, abrasion resistance and heat resistance on surface, extending service life. Thermal spraying coating has special performances, including thermal insulation, insulation, grind sealing, self-lubrication, thermal radiation and electromagnetic shielding; using thermal spraying can repair components.
7. Controlling corrosive environments
Environments are classified into generalized environments and narrow environments. Generalized environments refer to the surrounding environments of valve installation place and media flowing inside valve; narrow environments refer to the surrounding conditions of valve installation place. Most of environments are unable to be controlled. And production also cannot be changed randomly. Only under the circumstance that cannot cause damage on products or techniques, controlling corrosive environments can be used, for example, deoxidation in boiler water or adding alkali in refinery process to regulate PH value. Adding corrosion inhibitor and electrochemical protection also belong to controlling corrosive environments.
The atmosphere is filled with dust, vapor and smoke, especially in production environments such as poisonous gases and micro powders emitted by chimney and equipment, causing different corrosion on valves. Operating personnel should follow the regulation in operating instructions, cleaning, purging or cheering valve regularly. It is effective for controlling environmental corrosion. Stem installed with protection cover, valve set with well or spraying paint on surface are all methods used for corrupt prevention. Environmental temperature growing and air pollution accelerate corrosion on valves or equipment, especially under sealing environment. So, opened factory, ventilation or cooling measures should be adopted, slowing down environment corrosion.
8. Improving processing technique or structure of valves
Corrosion prevention of valves should be taken into consideration from design. Valves having reasonable structural design and correct technique are the best way for slowing down corrosion.
Therefore, design and manufacturing should improve the components which have unreasonable structure, false technique and can easily cause corrosion so that they can meet requirements of different working conditions.
The gap of valve connection is a good environment for causing oxygen-concentration cell corrosion. Threaded connection should not be adopt at the connection between stem and closure member as possible; valve welding should use double butt welding and it should be welded continuously well. Spot welding and overlap welding can cause corrosion. Threaded connection of valve should use adhesive tapes or pad made of PTFE. It not only has good sealing, but also has good corrosion resistance. Media in dead angle which cannot flow easily can corrode the valves easily. Besides not invert valve or discharging depositional media, depression structure should be avoided as possible when manufacturing valves. Drain holes also should be set on valves.
Different metals contacting can form galvanic couple, which can cause corrosion on anodic metals. So, when choosing metals, metals having large metal potential difference while cannot cause passive film should be prevented from contacting. During manufacturing and processing, especially stress corrosion caused by welding or heat treatment, processing methods should be improved. Annealing treatment and other relative prevention measures should be used after welding. Raise the surface roughness of stem and other valve parts. The higher surface roughness is, the higher corrosion resistance is. Improve processing technique or structure of packing or gasket. Using flexible graphite and plastic packing, or using gasket made of flexible graphite or PTFE, can enhance sealing performance and reduce the corrosion on stem and sealing face of flange.
