Why are valves subjected to salt spray testing?

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In industrial fluid control systems, valves are key actuating and shut-off components that ensure process safety and equipment integrity. Their operating environments are subject to various degradation factors such as high humidity, salt deposition, and chemical corrosive agents, including marine engineering platforms, chemical processing facilities, road infrastructure in areas where salt is spread in winter, and coastal storage and transportation systems. Chloride ions (Cl⁻) in salt spray environments have strong penetrating ability; they can destroy the passivation film on metal surfaces, leading to failure modes such as pitting, crevice corrosion, and stress corrosion cracking. As an accelerated environmental corrosion testing method, the core purpose of the salt spray test is to simulate the long-term corrosion behavior of valves in a saline atmospheric environment, thereby enabling a rapid evaluation of materials and coating systems. By placing the sample in a salt spray environment as specified by standards, it is possible to reproduce the corrosion effects that occur over months or even years in natural conditions within just dozens to hundreds of hours, thereby supporting material selection, process optimization, and verification of batch consistency. The value of the salt spray test lies in accelerating corrosion verification. This experiment accelerates the electrochemical corrosion process through continuous spraying under constant temperature and humidity conditions; chloride ions first adsorb onto the metal surface, disrupting the stability of the oxide film and thereby forming corrosion microcells. This mechanism can quickly reveal the weak points of materials or coatings, such as coating pores, uneven plating, and reduced corrosion resistance in the weld heat-affected zone. Secondly, salt spray testing plays an important role in quality control and risk avoidance. Once a valve fails due to corrosion, it can lead to the leakage of the medium, unplanned shutdown of the system, and even safety accidents. Taking offshore platforms as an example, valve corrosion and leakage can cause environmental pollution and cascading damage to the equipment. Evaluating the corrosion resistance of valves through salt spray tests helps to identify batches and manufacturing processes with stable resistance to salt spray, thereby reducing the likelihood of failure during operation and the associated maintenance costs. Furthermore, salt spray testing can also provide a scientific basis for process selection and material choice. Among the commonly used valve materials, 316 stainless steel has a significantly better resistance to pitting corrosion in saline environments due to the presence of molybdenum, as compared to 304 stainless steel ； For galvanized parts, the coating thickness and density directly determine the timing of the appearance of white rust and red rust. Salt spray testing enables a quantitative assessment of the corrosion resistance limits of different substrates, coatings, and organic or inorganic finishes, supporting design and process iterations. The salt spray testing for industrial valves is usually carried out in accordance with ISO 9227 \"Corrosion testing in artificial atmospheres – Salt spray test\" or GB/T 10125-2021 \"Corrosion testing in artificial atmospheres – Salt spray test\". Common test methods include the neutral salt spray test (NSS) and the copper-accelerated salt spray test (CASS). The neutral salt spray test is suitable for the routine corrosion resistance evaluation of most metals and their coatings, as well as finished valves ； The copper-accelerated acetate fog test involves adding 0.26 g/L of copper(II) chloride dihydrate to a 5% sodium chloride solution, with the pH being adjusted to 3.1–3.3 using glacial acetic acid; it is suitable for the rapid evaluation of stainless steel or coated valves that require high corrosion resistance. The control requirements for the key test parameters are as follows: the concentration of the sodium chloride solution is 50 g/L ± 5 g/L (i.e., 5% ± 0.5%) ； The spray rate is 1.5 mL/h to 2.5 mL/h per 80 cm² of horizontal collection area ； The test temperature is 35 ℃ ± 1 ℃ ； The pH value of the collection solution in the neutral salt spray test should be controlled between 6.5 and 7.2. The spray pressure and atomization degree must be such that the salt mist settles evenly, without directly impacting the sample surface. A common misconception is that salt spray testing is unnecessary as long as a corrosion-resistant alloy such as 316 stainless steel is used for the valve substrate. In practice, processing steps such as welding, grinding, and surface treatment may damage the local microstructure of the material or the integrity of its coating. The weld area may suffer preferential corrosion due to compositional segregation in the heat-affected zone or residual stresses ； If the coating or plating has pores, insufficient thickness, or poor adhesion, typical areas of preferential anodic dissolution will form. Salt spray testing can effectively identify the aforementioned process defects, which are difficult to detect through routine visual inspections or dimensional checks. After the salt spray test, the valve samples should be evaluated in accordance with standards or product technical specifications. Common evaluation criteria include: the area and distribution of corrosion in the base metal and weld zone (which can be rated according to GB/T 6461 or ISO10289aa) ； Bubbling, peeling of coatings or platings, and the width of corrosion spread at scratches ； Functional integrity of the valve, such as changes in opening and closing torque, condition of the sealing surfaces, and flexibility of valve stem operation ； As well as the corrosion depth in local areas such as threads and flange sealing surfaces. Only if it meets the specified corrosion resistance grade and functional requirements after testing can it be deemed qualified. In summary, the salt spray test is not a mere formal inspection procedure; rather, it is an important technical method for verifying a valve’s environmental resistance. Its core value lies in identifying hidden defects in materials, coatings, and manufacturing processes in advance through standardized accelerated corrosion conditions, thereby providing quantitative data support for product design and quality control. In internal technical training, emphasis should be placed on mastering the principles of testing, standard parameter control, sample evaluation methods, and rules for interpreting results, in order to avoid relying on empirical statements or unverified performance data. Scientifically conducting salt spray tests helps reduce the risk of corrosion-related failure of valves under harsh operating conditions, ensuring the long-term safe operation of industrial systems.