The filiform corrosion chamber is a device used for accelerated corrosion testing. Its main function is to simulate the corrosion process of metal surfaces in atmospheric environments. Through this equipment, the corrosion behavior and corrosion resistance of materials in specific environments can be quickly obtained.
Ⅰ. Understanding filiform corrosion
Filiform corrosion has been observed under thin organic coatings such as aluminum, steel, aircraft structures exposed to humid atmospheres, beverage cans, flanges, gaskets, and weld zones of debonded base metals coated with organic coatings. Corrosion-resistant alloys of stainless steel, copper, and titanium are not susceptible to filiform corrosion. Figure 1.12 shows filiform corrosion of a steel substrate coated with a transparent acrylic paint [38]. Filiform corrosion was induced on a knife-scratched specimen exposed to 5% NaCl and humidity (40% and 80% RH). The corrosion of the base metal can be clearly observed through the transparent paint surface. The line in the middle represents the knife scratch. The filiform corrosion grows in two directions from the line and consists of a head and a tail. The head and tail are filled with FeCl₂ solution and corrosion products.
Filiform corrosion is an all-round, thread-like (hair, worm-like) attack on the metal beneath the coating, resulting in localized cracking due to the formation of corrosion products. This attack occurs beneath coatings of lacquer, varnish, and paint, particularly on steel (e.g., coated steel tanks), aluminum (e.g., in the construction industry, often used in aircraft, and sometimes in aluminum packaging foil), and magnesium. Filiform corrosion can also occur beneath springs made of polyurethane, linseed oil, various alkyd resins, urea resins, and epoxy resins.
In aircraft, this type of corrosion is particularly prevalent during flights of several hours in hot climates, during patrol aircraft flying low over the ocean, during multiple transoceanic flights, or during stationary operations at coastal airports. This is caused by elevated salt concentrations in the atmosphere. Besides outdoor air, filiform corrosion can also occur in humid indoor environments.
Besides being unsightly, filiform corrosion generally does not have catastrophic consequences due to its limited depth of attack. Aluminum packaging foil is an exception, where it can corrode completely, rendering the packaged goods no longer airtight and moisture-proof. In other cases, filiform erosion of the coating can lead to further corrosion of the metal.
Ⅱ. Technical Principle
The filiform corrosion test chamber is primarily based on the principle of electrochemical corrosion. In a natural environment, metal surfaces react with oxygen and moisture in the air to form a galvanic cell, leading to metal corrosion. The filiform corrosion test chamber accelerates this process in the following ways:
1. Humidity and Temperature Control: The chamber is equipped with a constant temperature and humidity system that precisely controls the temperature and humidity within the chamber, providing a stable corrosion environment. This helps accelerate the electrochemical reactions on the metal surface, thereby accelerating the corrosion process.
2. Salt Spray Corrosion: The chamber can spray a certain concentration of salt spray to simulate marine or other high-salinity environments. The sodium chloride in the salt spray reacts with the metal surface, accelerating the corrosion process.
3. Electrochemical Measurement: The built-in electrochemical testing system monitors electrochemical parameters of the metal surface, such as corrosion current and polarization resistance, in real time. This data is valuable for evaluating the corrosion resistance of materials.
4. Adjustable Gas Environment: The chamber's gas composition can be controlled, such as by increasing the concentration of carbon dioxide or sulfur dioxide, to simulate different industrial or polluted environments.
Ⅲ. Advantages
1. High Efficiency: The test chamber can simulate long-term natural corrosion processes in a short time. This is of great significance for material R&D and quality testing, significantly shortening the product development to market cycle.
2. High Controllability: By precisely controlling the temperature, humidity, salt spray concentration, and gas composition within the test chamber, the corrosion environment can be precisely controlled. This makes test results more reliable and reproducible.
3. Accurate Data: The built-in electrochemical measurement system monitors and records key parameters during the corrosion process in real time, providing accurate data support for subsequent data analysis.
4. Wide Application: The test chamber is not only suitable for corrosion research of metal materials, but can also be applied to coatings, electronic components, automotive parts, and other fields. Its versatility makes it a valuable tool in materials science research.
5. Safety and Reliability: The test chamber is designed with multiple safety protection measures, such as temperature alarms and overload protection, to ensure safe and stable operation. Furthermore, the closed test environment prevents external factors from interfering with experimental results.
Ⅳ. Factors Affecting Filiform Corrosion
Temperature
Based on the mechanism of filiform corrosion, temperature and relative humidity are the primary factors affecting it. As temperature increases, molecular motion intensifies, accelerating the corrosion rate. Arrhenius's law states that the reaction rate increases by 2-3 times for every 10°C increase in temperature. Furthermore, increasing temperature increases the conductivity of the electrolyte solution, thereby accelerating the electrochemical corrosion reaction.
Relative Humidity
Filiform corrosion generally occurs most readily in ambient relative humidity (RH) between 60% and 95%. However, when hygroscopic salts, such as chloride salts, are present on the coating surface, filiform corrosion can occur even at lower RH. However, when RH exceeds 95%, filiform corrosion slowly grows and ceases. This is because the oxygen content in the air decreases at RH, significantly slowing the corrosion reaction.
Oxygen Content
Research indicates that oxygen content is the primary factor affecting filiform corrosion. Related studies have shown that an oxygen volume fraction of 35% in the environment rapidly accelerates filiform corrosion, while an oxygen volume fraction of approximately 50% is most effective.
Other Factors
Temperature, relative humidity, and oxygen content are the most significant factors influencing filiform corrosion. Furthermore, the surface pretreatment method of the coated base metal, the coating system, and atmospheric contaminants all significantly influence filiform corrosion. For example, phosphating with zinc phosphate exhibits better filiform corrosion resistance than phosphating with iron phosphate, while Ti-Zr passivation pretreatment exhibits weaker filiform corrosion resistance than chromating pretreatment.
Ⅴ. Test process
(1) Sample scratch
Use a scratching tool to scratch a vertical "丨" line on the sample surface. The length is recommended to be 100mm, at least 40mm, and at least 15mm away from the edge. The scratch direction should be consistent with the placement direction of the sample in the box. After scratching, use a multimeter to check whether there is a continuous current to check whether the sample has been scratched to the metal substrate. If it has not completely reached the substrate, it is necessary to scratch again at least 15mm away from any scratch or edge.
(2) Salt spray initiation
Place the scratched sample in a CASS test chamber under ASTM B 368 standard conditions for 6 hours. Place the sample surface and the scratch at a 45° angle on a non-metallic sample holder. This process mainly uses the salt spray corrosion environment to expose the sample to the corrosive atmosphere and form a "growth initiation point" at the scratch.
(3) Deionized water cleaning
After salt spray initiation, prepare a container of at least 5 gallons and fill it with flowing deionized water (ASTM D 1193.type 4). Immerse the sample vertically in the flowing deionized water, turn 90° left and 90° right in the horizontal direction, return to the original position, and then take it out vertically to remove the excess CASS solution carried on the surface of the paint film. The whole process takes about 3 seconds.
(4) Wet and hot placement
Place the sample at a 45° tilt angle in a constant temperature and humidity test chamber with a temperature of (65±1)℃ & air humidity of (85±3)%RH & air flow of (6~24m/min) for 672 hours. Check the growth of filiform corrosion on the surface of the sample every 168 hours. During the mid-term inspection, the time when the sample is taken out of the humidity chamber shall not exceed 15 minutes.
Ⅵ. Corrosion Results Evaluation
After the test, the filiform corrosion results are evaluated. The 5mm at each end of the scratch is disregarded and not included in the evaluation range. The corrosion length must be measured perpendicular to the scratch, not along the direction of corrosion propagation, and the scratch itself should be avoided during measurement. The maximum corrosion length on either side of the scratch is recorded, and any other areas of the scratch are examined for filiform corrosion.
Material corrosion not only causes significant economic losses but also results in significant waste of metal resources and energy. It can also cause environmental pollution and significantly hinder sustainable development. Therefore, scientific research on corrosion is of great significance. While filiform corrosion is typically primarily a cosmetic issue at first, its development can damage the integrity of the coating (e.g., delamination and blistering), allowing further moisture ingress and leading to more severe corrosion problems. Filiform corrosion can undermine coating adhesion, causing delamination and loss of protective properties, exacerbating further corrosion damage to the substrate. Furthermore, filiform corrosion areas can cause stress concentration, accelerating corrosion fatigue and resulting in a decrease in strength. Therefore, effective prevention of filiform corrosion is crucial, both in substrate surface treatment and in the application of protective coatings. Through laboratory filiform corrosion accelerated tests, the corrosion phenomena in actual applications can be effectively simulated, thereby providing technical support for product research and development, process improvement and product quality control.
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