The effects of highly corrosive fluids on valves and disasters that can be avoided
Some of the highly corrosive fluids in the oil and gas industry contain the following compounds: carbon dioxide (CO2), hydrogen sulfide (H2S), chlorides and moisture. These corrosive fluids have different chemical interaction mechanisms with the valve’s body and demonstrate distinct behavioral tendencies when in contact with specific metallic materials. In crude oil, the concentration of H2S is between 0.05% and 15%. Sulfide concentrations above 0.15% are sufficient to cause sulfidation corrosion in plain steels and low-alloy steels.
CO2 becomes corrosive after its reaction with moisture, where it converts into carbonic acid. The kinetics of this reaction becomes favorable for the formation of carbonic acid (H2CO3) once partial pressure of CO2 reaches 0.5 bar, which is often the case in the industry. H2CO3 is a weak acid, and it becomes aggressive in acidic regions when it attacks steel, creating iron carbonate. Lastly, the chloride ions and their concentration in the pipeline should be accurately determined since they have one of the fastest adsorption rates to the steel surface. Once they penetrate through the coating layer, their reaction with metal oxides is very rapid. As a result, chlorides often cause a localized breakdown, a corrosion mechanism commonly known as pitting.
Corrosion-resistant check valve materials
One of the optimal anticorrosive materials for a valve’s body (such as the industrial check valve shown in FIG. 1) is stainless steel 316, due to the 2%–3% molybdenum content that improves 316’s corrosion resistance. Unreactive, non-metallic materials are suitable for other wetted components, such as valve seals, seats or linings. Piping systems with high corrosion rates due to the availability of chloride traces in the fluid use valves made from alloys with higher pitting resistance equivalent numbers (PREN). Alloys with higher PREN values are more resistant to corrosion than steel.
FIG. 1. The use of optimal anticorrosive materials, such as stainless steel 316, is suggested for equipment like industrial check valves.
Any severe service valve must provide sufficient sealing throughout its service life. While graphite-based packing materials provide excellent sealing at higher temperatures, they are likely to undergo galvanic corrosion. An alternative to such corrosive flow operations is using polytetrafluoroethylene (PTFE) valve packings.
Monel, an alloy of nickel, has better corrosion and chemical resistance compared to steel-based valve materials. Other alloys that perform well in corrosive and aggressive fluid service are Alloy C-276, which is commonly referenced as Hastelloy C-276 and is composed of nickel-molybdenum-chromium alloys and tungsten. Inconel is an alloy of nickel and chromium and resists oxidation at high temperatures and pressure.
Corrosion effects upon check valves
The safety of severe service pipelines relies on the integrity of check valves. These valves guarantee directional and volumetric control of hazardous fluids while preventing leakage. When corrosion occurs, the valves weaken, increasing the probability of fluid media leakages and fugitive emissions of toxic gases. Valve components consist of dissimilar materials (metals and non-metals) that react differently to the corrosive service media. Valve corrosion occurs in any or a combination of the following ways.
General corrosion. This occurs due to chemical or electrochemical reactions within the valve. Concentrations of sulfur, chlorides or carbonates mix with moisture and oxygen in the piping system to form weak acids that attack metal surfaces in a process called chemical corrosion. Elevated temperatures speed up the chain of chemical reactions, causing a rapid deterioration of the valve body. Electrochemical corrosion occurs when dissimilar valve materials come into contact with electrolytes and initiate a flow of electrons that causes gradual damage to metallic surfaces. The different metals form galvanic pairs: the strong metal is the cathode and the weaker one is the anode. The anode corrodes faster than the cathode. The rate of electrochemical corrosion in valves depends on the pH concentration of the service media (alkalinity or acidity) and the amount of oxygen in the pipeline. Low oxygen levels increase the rate of flow-accelerated electrochemical corrosion.
Pitting and crevice corrosion. Valves made from steel contain a protective oxide layer that inhibits chemical reactions. Exposing the valves to corrosive, aqueous fluid media initiates oxidation and causes the ferrous metal to lose some electrons, leading to the formation of small pits, which expand as the valve cycles increase. Crevice corrosion is predominant in valve connectors and welded joints. Crevices, which are as small as 0.025 mm–0.1 mm, permit the leakage of corrosive electrolyte solutions to form a galvanic cell that creates perfect conditions for localized corrosion (FIG. 2).
FIG. 2. Crevices permit the leakage of corrosive electrolyte solutions to form a galvanic cell, which creates perfect conditions for localized corrosion.
Frictional corrosion. Also known as erosion corrosion, this type of corrosion occurs when a viscous and corrosive fluid repeatedly passes through the valve at high speed. The viscous service media leaves behind small debris around the valve, scouring the metallic surface and gradually wearing out the protective oxide layers. As frictional wear continues, the underneath steel components are exposed to the corrosive media, increasing the rates of chemical and electrochemical attacks on the valve. Choke valves installed in oilfields to balance the pressure of the common manifolds are prone to this type of corrosion due to the aggressive nature of the oil well stream (mixture of oil, gases, semi-solids and water).
Stress corrosion cracking. Varying the operating temperatures of oil and gas processes causes expansion and contraction of valves and other components of the piping system. With time, these variations cause minute cracks on internal valve components, escalating check valve corrosion.
When check valves corrode, their performance drops. Frictional corrosion reduces the thickness of the flapper or disc. It also affects the geometry of the valve seat, leaving behind irregular seat surfaces and increasing the probability of leakage from valves. As the structural integrity of the disc deteriorates, its shutoff characteristics degrade. As the valve’s shutoff characteristics degrade and its disc loses efficiency, it becomes susceptible to upstream fluid flow even at pressures well below the designed cracking pressure.1,2 Because these factors lead to delays in valve closures, they also leave room for small backflows. Such instances result in downstream contamination and other adverse effects, such as water hammers and cavitation. Weakened discs/flappers frequently slam against the valve seats, with significant losses in the sealing capabilities.
Pitting, crevice corrosion and general corrosion attack the disc/flapper, valve stem and valve packing. As the corrosive action of the fluid attacks the stem, the diametral tolerance between the valve stem and the packing increases. This creates conditions for leakages and fugitive emissions around the valve body, escalating operational safety risks. Fluid leakages result in increased pressure drops across the valve, a situation that will affect the efficiency of downstream processes.3 Over time, the corrosive fluids attack the external components like the actuators, affecting the responsiveness of valves to process controls.
As rust accumulates around the valve, the opening and closing limits of the valves reduce. The check valves cannot provide a full path for fluids to pass. At the same time, the process operating conditions are kept constant, with the pumps supplying fluids at optimum capacity. The partially open check valve disc obstructs the flow of corrosive fluids, resulting in a rapid buildup of pressure in the upstream piping equipment. With time, the excess fluid pressure weakens the pipes, causing rupture and explosion.
Prevention and maintenance
Selecting the appropriate valve for corrosive fluid service eliminates safety risks arising from the gradual degradation of valve components. One way to protect valves used for corrosive fluid service is by performing metal surface treatment. A protective, non-reactive coating is applied between a metal surface and its surroundings to create a physical barrier. This prevents chemical reactions at the surface of the metallic component. Other treatment methods include surface coating, passivation or oxidation. The protective coatings use corrosion-resistant materials and alloys like chromium, zinc, metal oxides and phosphates.4,5 To protect valves against electrochemical corrosion, manufacturers use anodic or cathodic protection, which converts galvanic pairs into passive components to prevent electron flow. Other valves use non-metallic corrosion inhibitors like ceramics, metal oxide ceramic coatings and non-metal substrates. To combat and inhibit corrosion, operators should remain vigilant in their inspection and maintenance procedures (FIG. 3).
FIG. 3. To combat and inhibit corrosion, operators should remain vigilant in their inspection and maintenance procedures.
Heavy-duty, severe service valves contain ports for the lubrication of internal valve surfaces to enable the smooth movement of dynamic parts. This reduces the impact of frictional corrosion.
Corrosion predominantly attacks a check valve’s disc, as it is the most dynamic component. To minimize the impacts of corrosive chemical attacks on the check valve discs, they are often coated with 0.3 mm–0.6 mm of ethylene chlorotrifluoroethylene (Halar) material. Another way to protect the discs is by coating metallic valve components with polyethylene or perfluoroalkoxy (PFA) materials. These thermoplastic materials possess better mechanical and chemical resistance than Halar.
The choice of valve material varies depending on the concentration of the dominant corrosive compounds. Piping systems with high concentrations of sulfur, with suitable conditions for the formation of sulfuric acid (H2SO4), should use valves made from alloy stainless steel, such as Alloy-20. Fluoroplastic valves can be used for low-pressure, low-temperature H2SO4 service.
Pipelines conveying fluids with less than 30% hydrochloric acid concentrations at temperatures below 120°F (50°C) can use valves made from steel alloys containing suitable quantities of molybdenum. Non-metallic valve materials are appropriate for hydrochloric acid service as long as the process temperatures do not exceed 300°F (150°C). High-temperature nitric acid service pipelines use titanium alloy valves. Nitric acid corrodes most metals at room temperature and pressure. Where there is a high concentration of H2S gas in the service media, the valve stem is electroplated using phosphorous and nickel coatings.
Non-metallic materials such as PTFE, synthetic rubbers, nylon and strong thermoplastics are used to create seals in valves. Although they provide sufficient sealing, their use is limited to low-temperature applications.
When installing non-metallic valves for corrosive fluid service, avoid distorting the flanges. The machining of non-metallic flanges is not as smooth as that of most metallic valves and can be a source of external leakages.
Always use the correct valve and piping materials that are resistant to the service media. Installing the valves in the appropriate locations and orientations prevents instances of choked flows, stagnation or contamination that can exacerbate the corrosive action of the working fluid. HP
LITERATURE CITED
Driscoll, R., “What is the cracking pressure of a check valve?” Valveman, online: https://valveman.com/blog/what-is-the-cracking-pressure-of-a-check-valve/
Wood, R., “Erosion-corrosion interactions and their effect on marine and offshore components,” U.S. Department of Energy, Office of Scientific and Technical Information, 2004, online: https://www.osti.gov/etdeweb/biblio/20671857
Zulkarnaini, M. A., et al., “Analysing petroleum leakage from ground penetrating radar signal,” ResearchGate, January 2019, online: https://www.researchgate.net/publication/325115239_Analysing_Petroleum_Leakage_from_Ground_Penetrating_Radar_Signal
Waters, R., “Selection of severe service valves,” Chemical Engineering, June 2017.
SpecialChem, “Corrosion resistance and anti-corrosion coatings—All you need to know,” SpecialChem: The material selection platform, online: https://coatings.specialchem.com/coatings-properties/corrosion-resistance
