When you’re buying fasteners—bolts, nuts, screws, etc.—you want to ensure that they’re high quality and that they’ll last over time. You depend on your fasteners’ durability and resilience. You want them to hold up, no matter which harsh environments they’re placed in or what elements they come in contact with. Rusted fasteners can be dangerous and costly to replace. To provide you with the durability that you require from your fasteners, manufacturers rely on the results of salt spray tests.

What It Is in a Nutshell

Salt spray testing is a method of checking the corrosion resistance of coatings and materials used to manufacture products like fasteners. The surface coatings are used in order to provide a level of protection for the metal underneath it. During salt spray testing, an accelerated corrosive attack is produced in order to better predict how well the coating protects the metal. The purpose of the testing is to evaluate a surface coating’s expected life.

Effectiveness

Salt spray testing is typically used to produce quick comparisons between expected and actual corrosion resistance. In reality, there is only a weak correlation between the coating’s duration in the test and its actual expected life. This is due to the fact that many external factors influence corrosion—it’s not a simple process. This is especially true for materials like hot dip galvanized steel, where drying cycles are an important factor to consider for durability.

For this reason, salt spray testing is most effective when used on samples to determine a pass-or-fail grade and compare it to expectations, typically in a quality audit role or to test for the effectiveness of a particular production process.

Salt spray tests are used on phosphate surfaces, zinc and zinc-alloy plating, electroplated tin, copper, nickel, and chromium, organic coatings, paint coatings, and coatings that aren’t applied electrolytically.

Why Do We Use It?

Salt spray testing has long been the standardized corrosion test method because it’s quick, repeatable, and relatively inexpensive. The first such internationally recognized test dates back to 1939. The ASTMB117 test is still the most popular salt spray test used worldwide today. ASTM B 117 and ISO 9227 tests are also standardized and used today.

How It Works

Salt spray tests are conducted in a closed testing chamber. A saltwater solution is applied to a sample via a spray nozzle. This dense salt water fog is used to imitate a corrosive environment. After a period of time, which is dependent on the corrosion resistance of a product, the appearance of oxides is evaluated. The more resistant a product is, the longer it takes for oxides to appear. Testing periods can take between eight to twenty-hour hours to complete for materials like phosphate steel to 720 hours for zinc-nickel coatings and even 1,000 hours or more for certain zinc flake surface coatings.

Different Types of Salt Spray Testing

A neutral salt spray test (NSS) using a 5 percent NaCl solution is most often used for steel-based materials. This type of solution has a neutral pH level of 6.5 to 7.2. On the other hand, decorative coatings, like anodized aluminum, electroplated copper-nickel, and electroplated copper-nickel-chromium are tested with a solution that includes chemicals like acetic acid (ASS) or acetic acid with copper chloride (CASS). These acidified solutions have a pH between 3.1 and 3.3.

The results of a material or coating’s resistance to corrosion in a salt spray test can help you determine whether or not a particular fastener is durable and resilient enough for your needs.