A 24-hour salt spray test is equivalent to how long would it take in a natural environment?
In production, we will encounter many products that require galvanizing, especially sheet metal structural parts, Precison CNC machining parts, etc. Good anti-corrosion performance is the key process to ensure the long-lasting and stable operation of the product
During the galvanizing process, sometimes our products pass the 24-hour salt spray test and rust after only three months of shipment. It must be due to poor quality from you! ”It is widely circulated in the industry that 24-hour salt spray is equivalent to one year of outdoor use. Whenever customers hold rusty samples to hold them accountable, sales can only apologize continuously, but engineers are puzzled. Although the third-party salt spray report was deemed PASS, the finished product was put into use but expired prematurely. Many people suspect that the salt spray equipment is malfunctioning or that there are loopholes in industry standards. In fact, the root of the problem lies in people's misunderstanding of the conversion relationship between salt spray duration and actual service life.
1. Do not blindly believe in the empirical formula that 24 hours of salt spray equals 1 year outdoors.
Many practitioners mistakenly believe that this conversion standard is the conclusion drawn from authoritative experiments in Europe and America, but the fact is not so. This ratio is only an empirical value summarized by engineers after actual testing in the early years. ISO 9227, ASTM B117, and domestic GB/T 10125-2021 have never recognized fixed conversion rates. The environment inside the salt spray box is very constant: 35 ℃ constant temperature, continuous spraying of 5% concentration salt water throughout the day; However, the real environment has a large temperature difference between day and night, accompanied by variables such as rainfall, ultraviolet radiation, and coastal salinity. The difference in the rate of accelerated corrosion is significant, and 24-hour salt spray in inland dry areas roughly corresponds to 2-3 years of natural environment; In the coastal areas of Meiyu, 24-hour salt spray may only last 2-6 months outdoors. So this experience value can only be used as a reference and cannot be used to promise customers a warranty period.
2. Putting aside the thickness of the coating and passivation quality, it is meaningless to talk about the service life.
However, achieving 24-hour neutral salt spray qualification still has a huge difference in the corrosion resistance of different coatings in the later stage. Blue and white galvanized parts, with a thin zinc layer and cheap passivating agents, will rust in coastal environments for about six months; The thick zinc layer combined with high-quality trivalent chromium sealed passivation can be used for 1-2 years in indoor environments and stable for more than 1 year in coastal environments; Single layer nickel plating and chrome plating are qualified for short-term salt spray, but pinhole corrosion will occur after a few months; The Cu Ni Cr three-layer coating is very common for daily use for 1-3 years; Electrophoretic spray coating products can be used indoors for 3-5 years after pre-treatment phosphating is in place. Once the oil removal does not meet the standard, water vapor will seep into the interior of the coating, and the surface will slowly peel off after the substrate rusts. The thickness of the coating, passivation agents, and electroplating process are the key factors determining the product's lifespan. Judging the quality of a product solely based on the duration of salt spray is itself a misconception.
3. Neutral salt spray can only be used to compare processes and cannot predict the actual service life.
Many factories use salt spray testing as the basis for determining product durability, which is a major cognitive misconception. The core function of neutral salt spray (NSS) is twofold: firstly, to inspect the uniformity of the coating, and to check for missed plating, pinholes, and surface scratches; The second is to compare the advantages and disadvantages of the process horizontally, and prioritize selecting suppliers with longer salt spray durability under the same conditions. The ordinary salt spray environment lacks wet dry alternation and ultraviolet irradiation, and the conditions are too idealized to accurately calculate the outdoor service life. This is like a normal physical examination result, which only represents a good current physical condition and cannot guarantee long-term health. If we want to truly evaluate the outdoor lifespan, we need to conduct alternating salt spray and ultraviolet aging tests, but the test cycle is long, the cost is high, and it is rarely implemented in the mass production stage.
4. There are four main reasons for the early rusting of mass-produced parts despite the qualified salt spray samples.
Firstly, inadequate pre-treatment, incomplete oil and rust removal, insufficient adhesion of the coating, and gradual erosion of the substrate by water vapor; Secondly, passivation agents are treated differently. High end passivation solutions are used for sample inspection, while cheap agents are selected for mass production batches to reduce costs. The passivation film ages rapidly in the later stage; Thirdly, slight scratches may occur during transportation, and laboratory samples are all selected from intact parts. Once the coating of mass-produced parts is damaged, corrosion will quickly spread; Fourthly, if the actual working conditions of the customer exceed expectations and verbal instructions are given for indoor use, the actual installation in high humidity and high salt areas such as kitchens and beaches will significantly shorten the anti-corrosion life.
5. In the face of customer doubts, this communication technique can reasonably resolve conflicts.
When customers demand a one-year warranty for the duration of salt spray, the response can be as follows: Mr. Wang, salt spray is only a laboratory acceleration test, mainly used to verify the stability of electroplating processes, just like car collision tests, which can only test the design level and cannot predict real accidents. There is a significant difference between the sealed laboratory environment and the site exposed to sunlight and rain. The product lifespan is closely related to the humidity and salt content on site. We strictly control the coating thickness according to industry standards and promise quality assurance based on the actual usage environment. Please rest assured. Don't dwell on the conversion period of salt spray, rely on the strength of the technology to communicate, it is much easier to gain customer recognition than to argue forcefully.
I have a deep understanding of the hardware electroplating industry: blindly relying on salt spray duration to estimate product lifespan can easily lead to conflicts with customers in the later stages. Salt spray is just a quality inspection tool, not a panacea for predicting good or bad. To produce reliable products, stable electroplating processes, standardized pre-treatment, qualified passivation agents, and improved packaging protection are essential; Confirming the usage environment and agreeing on the warranty scope with the customer in advance is the key to long-term operation.
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