In the realm of material science and product development, ensuring longevity and resistance to environmental degradation is paramount. Among the various threats, salt spray corrosion stands out as a significant concern, particularly for metallic components and products destined for harsh environments. At Dongguan Precision Test Equipment Co., Ltd., our salt spray test chambers play a crucial role in helping manufacturers understand and combat this corrosive force. Let's delve into the fundamental principles, applications, and influencing factors of salt spray testing.

1. The Fundamentals and Applications of Salt Spray Testing

1) The Detrimental Effects of Salt Spray Corrosion:

Salt spray corrosion poses a significant threat to various materials and products:

• Damage to Protective Coatings: It degrades metallic protective layers, leading to loss of aesthetics and reduced mechanical strength.
• Electronic Component Failure: Corrosion can interrupt power circuits in electronic components and wiring, a problem exacerbated in vibrating environments.
• Reduced Insulation Resistance: Salt spray deposited on insulators lowers surface resistance. Absorption of saline solution can decrease volume resistivity by several orders of magnitude.
• Seizure of Moving Parts: Corrosion products can increase friction in mechanical and moving parts, potentially leading to seizing.

2) The Mechanism of Salt Spray Corrosion:

Salt spray corrosion of metals is primarily an electrochemical process driven by the conductive salt solution penetrating the metal. This creates micro-galvanic cells with "low potential metal - electrolyte solution - high potential impurities." Electron transfer occurs, causing the metal (anode) to dissolve and form new compounds – corrosion products.

The same principle applies to metallic and organic protective layers. When the salt solution (electrolyte) permeates these layers, micro-galvanic cells form between the underlying metal and the protective layer.

The chloride ion (Cl-) is the primary culprit in this destructive process. Its strong penetrating ability allows it to easily pass through metal oxide layers and reach the underlying metal, disrupting its passive state. Furthermore, the chloride ion's low hydration energy allows it to be readily adsorbed onto the metal surface, displacing oxygen in the protective oxide layer and initiating corrosion.

Beyond chloride ions, dissolved oxygen in the salt solution (present as a thin film on the test sample) significantly influences the corrosion mechanism. Oxygen acts as a cathodic depolarizer, accelerating the anodic dissolution of the metal. The continuous salt spray during testing ensures a near-saturated oxygen level in the salt film. The formation of corrosion products can also cause volume expansion within metal defects penetrated by the salt solution, increasing internal stress and leading to stress corrosion and blistering of protective layers.

(Diagram of a typical Salt Spray Test Chamber)

3) What is Salt Spray Testing?

Salt spray testing is an accelerated corrosion test that utilizes a salt spray test chamber to create an artificial salt fog environment. This controlled environment allows for the evaluation of the corrosion resistance of products or metallic materials by observing changes in their properties after exposure and assessing their ability to withstand salt water spray.

Products commonly subjected to salt spray testing are primarily metallic, with the aim of evaluating their resistance to corrosion. Examples include steel wire ropes, high-zinc iron wires, and steel plates.

4) Classification of Salt Spray Tests:

Laboratory-simulated salt spray tests are broadly categorized into three main types:

• Neutral Salt Spray Test (NSS Test): The oldest and most widely used accelerated corrosion test. It employs a 5% sodium chloride (NaCl) solution with the pH adjusted to a neutral range (6.5-7.2). The test temperature is consistently 35°C, with a required salt spray deposition rate of 1-2 ml/80cm²/h.
• Acetic Acid Salt Spray Test (ASS Test): An evolution of the NSS test, it involves adding glacial acetic acid to the 5% NaCl solution, lowering the pH to around 3, resulting in an acidic salt fog. Its corrosion rate is approximately three times faster than the NSS test.
• Copper-Accelerated Acetic Acid Salt Spray Test (CASS Test): A more recent rapid salt spray corrosion test, primarily used in some regions. The test temperature is elevated to 50°C, and a small amount of copper salt (cupric chloride) is added to the salt solution, strongly inducing corrosion. Its corrosion rate is roughly eight times faster than the NSS test.
• Minimum Test Duration: Typically starts from 16 hours.

5) Factors Influencing Salt Spray Test Results:

Several key factors significantly impact the outcome of salt spray tests:

• A. Test Temperature and Humidity: Temperature and relative humidity directly influence the corrosive action of salt fog. The critical relative humidity for metal corrosion is around 70%. At or above this level, salt deliquesces to form a conductive electrolyte. Below this, the salt solution concentrates until crystallization occurs, slowing down corrosion. Higher test temperatures accelerate the corrosion rate. The International Electrotechnical Commission (IEC) standard IEC60355:1971 suggests that the corrosion rate increases by 2-3 times for every 10°C rise in temperature, and the electrolyte conductivity increases by 10-20%. For NSS testing, 35°C is generally considered appropriate. Excessively high temperatures can alter the corrosion mechanism compared to real-world conditions.
• B. Salt Solution Concentration: The effect of salt concentration on corrosion rate depends on the material and coating type. For steel, nickel, and brass, corrosion rate increases with concentration up to 5%. Beyond this, the rate tends to decrease, likely due to the relationship between salt concentration and dissolved oxygen levels in the solution. Oxygen acts as a cathodic depolarizer. However, for metals like zinc, cadmium, and copper, the corrosion rate generally increases consistently with increasing salt concentration.
• C. Sample Placement Angle: The angle at which the sample is placed significantly affects the results. Salt fog deposition is primarily vertical. Horizontal placement maximizes the surface area exposed to the spray, leading to the most severe corrosion. Studies have shown a significantly higher corrosion rate on steel plates placed at a 45-degree angle to the horizontal compared to vertically placed plates. Standard GB/T2423.17-93 specifies a 30-degree angle to the vertical for flat samples.
• D. Salt Solution pH Value: The pH of the salt solution is a crucial factor. Lower pH values indicate higher hydrogen ion concentration, increased acidity, and thus greater corrosivity. Salt spray tests on electroplated parts like Fe/Zn, Fe/Cd, and Fe/Cu/Ni/Cr have shown that the ASS test (pH 3.0) is 1.5-2.0 times more corrosive than the NSS test (pH 6.5-7.2). Maintaining a stable pH value within the specified range in salt spray standards is essential for ensuring the reproducibility of test results.
• F. Test Duration: The length of the exposure period directly correlates with the extent of corrosion. Longer test durations are used to assess long-term corrosion resistance.

2. Testing Standards:

Common standards for salt spray testing include ISO 4628.3, GB/T 2423.17, IEC60068-2-11, ASTM B117, JIS-Z2371, JIS-G3141, GJB 150.1, MIL-STD-810F, MIL-STD-883E, and more. Specific examples include:

• GB/T 2423.17-2008: Environmental testing for electric and electronic products - Part 2: Testing methods - Test Ka: Salt mist
• IEC 60068-2-11:1981: Basic environmental testing procedures - Part 2: Tests - Test Ka: Salt mist
• ASTM B117-11: Standard Practice for Operating Salt Spray (Fog) Apparatus
• ASTM B368-09: Standard Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test)
• GB/T 10125-1997: Artificial atmospheres - Corrosion tests - Salt spray tests
• ISO 9227:2006: Corrosion tests in artificial atmospheres - Salt spray tests
• GB/T 2423.18-2000: Environmental testing for electric and electronic products - Part 2: Testing methods - Test Kb: Salt mist, cyclic (sodium chloride solution)
• IEC 60068-2-52:1996: Environmental testing - Part 2: Tests - Test Kb: Salt mist, cyclic (sodium chloride solution) 
   

3. Evaluation Methods:

The purpose of salt spray testing is to evaluate the corrosion resistance quality of products or metallic materials. The accurate and reasonable determination of salt spray test results is crucial for correctly assessing a product's resistance to salt spray corrosion. Common evaluation methods include:

• Rating Method: The percentage of corroded area to the total area is categorized into several levels, with a specific level serving as the acceptance criterion. This method is suitable for evaluating flat samples.
• Weight Loss Method: The corrosion resistance quality of a sample is assessed by calculating the weight loss after the salt spray test. This is particularly applicable for evaluating the corrosion resistance of specific metals.
• Appearance of Corrosion Products Method: A qualitative method where the presence or absence of corrosion phenomena on the product after the salt spray test determines the pass/fail criteria. This method is frequently used in product standards.
• Corrosion Data Statistical Analysis Method: This method provides tools for designing corrosion tests, analyzing corrosion data, and determining the confidence level of the data. It is primarily used for analyzing and statistically evaluating corrosion patterns rather than direct quality judgment of a specific product.

Partnering with Dongguan Precision for Robust Corrosion Resistance Testing:

Understanding the principles, influencing factors, and evaluation methods of salt spray testing is crucial for manufacturers seeking to ensure the durability and longevity of their products in corrosive environments. At Dongguan Precision Test Equipment Co., Ltd., our state-of-the-art salt spray test chambers and our expertise in environmental testing can provide you with the accurate and reliable data you need to develop and deliver corrosion-resistant products. Contact us today to discuss your specific salt spray testing requirements and learn how our solutions can benefit your quality assurance processes.