Have you ever wondered why stainless steel castings rust after shipping? Have you ever encountered a situation where stainless steel castings were covered in brown or red stains after being unpacked, forcing you to return them for recasting? This occurs because the castings did not undergo standard salt spray testing before leaving the factory, making them vulnerable to corrosion and unable to withstand chloride-ion pitting during shipping. This article will explain the importance of salt spray testing for stainless steel castings and how it can help reduce your costs.
What is Salt Spray Testing for Investment Castings?
Salt spray testing in the stainless steel investment casting process artificially simulates a salt spray environment by atomizing a 5% NaCl solution into a fine mist within a sealed chamber, allowing salt particles to settle naturally on the casting surface to determine whether the corrosion resistance and surface treatment meet the required standards. The purpose of the test is to accelerate the simulation of how castings perform in real environments over several dozen hours, thereby verifying whether stainless steel castings meet the required performance standards, including material properties, heat treatment, and corrosion resistance.
Types of Salt Spray Test for Investment Castings
There are different types of salt spray tests for stainless steel investment castings depending on their specific application environments. Knowing these differences will help you select the appropriate salt spray test method based on your casting’s operating environment. This prevents the incorrect selection from failing to reflect the casting’s properties in real-world conditions, thereby failing to reveal potential pitting corrosion risks within the casting and leading to rapid corrosion and cracking during operation, resulting in unnecessary recasting costs.
- Neutral Salt Spray
This is the most common testing method for stainless steel castings. By simulating corrosive environments such as seawater and atmosphere, it evaluates general corrosion resistance and the integrity of the passive film in 304, 316, and other stainless steel castings to meet the required standards.
- Acetic Acid Salt Spray
The corrosion rate is three times that of the neutral salt spray test. It is primarily used to determine whether the properties of electroplated stainless steel castings meet standards in harsh environments, such as acidic industrial environments.
- Copper-Accelerated Acetic Acid Salt Spray
As an extremely accelerated test, the corrosion rate is 7 to 8 times that of the neutral salt spray test and is rarely used for ordinary stainless steel castings. It is commonly used for sampling on electroplated stainless steel castings or components subject to high corrosion requirements.
Key Factors Affecting Salt Spray Performance
Salt spray testing evaluates the performance of stainless steel castings under real-world operating conditions. As a result, it reveals potential issues at every stage of the investment casting process. Subjecting your stainless steel castings to salt spray testing allows you to determine whether the manufacturer’s operations meet required standards, enabling you to address quality issues early and avoid losses such as returns and recasting.
Chemical Composition
The duration of stainless steel castings in a salt spray environment is related to their corrosion resistance, which is closely linked to the passivation film on their surface. The formation of this passivation film depends on the carbon, molybdenum, and chromium content in stainless steel castings.
- Carbon Content
The higher the carbon content in a stainless steel casting, the more easily it combines with the chromium within the casting to form chromium carbides. This leads to chromium-depleted zones at the grain boundaries. The lack of chromium will significantly increase the possibility of intergranular corrosion and shorten the duration in salt spray testing.
- Chromium Content
Chromium content is the key to forming the chromium oxide passivation film. It is typically required to be ≥10.5% to form a dense, uniform passivation film capable of withstanding salt- spray erosion.
- Molybdenum Content
Molybdenum ions are the key to resisting pitting corrosion. For example, 316 stainless steel castings contain 2%–3% molybdenum, so their salt spray resistance is far greater than that of 304 stainless steel castings. However, if the molybdenum ion content drops significantly, the casting duration in salt spray testing will also be greatly reduced.
Solution Annealing
Solution heat treatment also significantly affects the duration of salt-spray testing of stainless steel castings. This is because if the casting has not undergone high-temperature treatment at 1050°C–1100°C and rapid water quenching, the carbides precipitated during cooling cannot be eliminated. This leads to chromium depletion at the grain boundaries, which significantly reduces the casting’s corrosion resistance, causing extensive rusting to occur within a few hours in a salt spray environment.
Casting Defects
During the pouring and cooling of stainless steel castings, improper handling can easily lead to internal defects such as porosity, shrinkage, and cracks. These defects can serve as critical triggers for corrosion during salt spray testing. When salt spray particles are applied to the surface, chloride ions will continuously accumulate in these pores, causing crevice corrosion in the castings and significantly reducing their service life during salt spray testing.
Surface Finish
Although investment casting can bring extremely low surface roughness to stainless steel castings, if the castings are not subjected to standard shot blasting and polishing, microscopic pits and burrs will remain on the surface, increasing surface roughness. This makes these areas highly susceptible to the accumulation of salt spray liquid during salt spray testing, leading to rapid corrosion.
Free Iron Contamination
If carbon steel grit is used for sandblasting stainless steel, tiny carbon steel particles will be forced into the surface of the stainless steel casting. Additionally, grinding the stainless steel casting with cutting tools that have previously cut carbon steel will also bring foreign iron particles onto the casting’s surface. These free iron ions will oxidize and rust first, generating free- iron rust on passivated stainless steel during salt spray testing. This creates the false appearance that the stainless steel casting is rusting, significantly influencing the results and may even lead you to spend a great deal of money to recast a casting that is actually defect-free.
Post Processing
In salt spray testing, the duration of stainless steel castings is related to the passivation film on their surfaces, which is fragile at first. If standard post-processing procedures, such as pickling and passivation, are not used to remove the scale from the casting’s surface and regenerate a dense passivation film, the casting will rust quickly during salt spray testing.
How to Improve Salt Spray Resistance of Stainless Steel Castings
Optimizing the investment casting process can effectively improve the salt spray resistance of stainless steel castings. Knowing which processes will help you optimize the salt spray resistance of your castings at minimal cost, avoid using more expensive materials, and significantly reduce component procurement costs.
Control the Composition
Use a spectrometer to ensure that the internal chemical composition is within limits before casting, which can improve the salt spray resistance of stainless steel castings from the source. This allows control of the stainless steel’s corrosion resistance at the source, thereby ensuring its salt spray resistance.
- Carbon Content
For ordinary stainless steel grades such as 304 and 316, the carbon content must be kept below 0.08%; for low-carbon stainless steel grades such as 304L and 316L, it must be kept below 0.03%.
- Chromium Content
Chromium ions are crucial to ensuring that the passivation film on the stainless steel surface is dense and uniform. To ensure that the chromium content meets the required standards, it must be maintained at 18%.
- Molybdenum Content
Molybdenum ions are key to the pitting corrosion resistance of 316 and 316L stainless steels; their content must be maintained between 2% and 3% to ensure the pitting corrosion resistance of stainless steel castings.
Standard Heat Treatment
Standardizing the heat treatment process is a critical step in addressing intergranular corrosion and reduced salt spray resistance in stainless steel castings. Proper heat treatment involves controlling the solution treatment temperature between 1050°C and 1100°C, with a holding time of approximately 1 hour. After the holding period, the casting must be transferred to the water quenching tank as rapidly as possible. This process eliminates precipitated chromium carbide from the stainless steel casting, thereby enhancing its resistance to intergranular corrosion and salt spray resistance.
Professional Post-Processing
Professional post-processing is key to enhancing the salt spray resistance of stainless steel castings. Professional pickling and passivation for stainless steel investment castings can thoroughly remove oxides left on the casting surface during cooling and regenerate a new, dense, and uniform passivation film, which directly improves the casting’s corrosion resistance and increases the salt spray resistance duration of 304 stainless steel castings by 2 to 4 times.
Electropolishing stainless steel castings can further enhance their salt spray resistance. It removes burrs from the casting surface and significantly reduces surface roughness, preventing the accumulation of salt spray residue. This protects the passive film from damage, enabling 316 stainless steel castings to withstand salt spray for 500–1,000 hours.
Performance Comparison
Controlling the chemical element content of castings and standardizing heat treatment and post-processing procedures can effectively improve the salt spray resistance of stainless steel castings.
The following table uses austenitic stainless steel as an example, and under the standard of ASTM B117 neutral salt spray, categorizes the condition as “unoptimized”, “full-process optimization,” and “Top Optimization”. This will give you a clearer understanding of how these professional treatments enhance the salt spray resistance of stainless steel castings.
| Grade | Condition | ASTM B117 NSS | Test Result |
| 304(CF8) | Unoptimized (Raw Cast, No Passivation) | 12-24 hours | High susceptibility to red rust and pitting corrosion |
| 304(CF8) | Full-process optimization (Full Process + Pickling/Passivation) | 96-120+ hours | Forming a passivation film, preventing red rust |
| 316(CF8M) | Unoptimized (Raw Cast, No Passivation) | 48-72 hours | Susceptible to pitting corrosion, and red rust is likely to form in hard-to-reach areas. |
| 316(CF8M) | Full-process optimization (Full Process + Pickling/Passivation) | 240-300 hours | Good resistance to pitting corrosion, meeting most industrial requirements |
| 316(CF8M) | Top Optimization (+ Electropolishing) | 500-1000+ hours | Extremely smooth surface, meeting marine-grade standards |
| 316L(CF3M) | Unoptimized (Raw Cast, No Passivation) | 48-96 hours | High possibility of intergranular corrosion, and red rust is likely to occur in polished areas. |
| 316L(CF3M) | Full-process optimization (Full Process + Pickling/Passivation) | 300-480 hours | Eliminates the possibility of intergranular corrosion |
| 316L(CF3M) | Top Optimization (+ Electropolishing) | 800-1500+ hours | Mirror surface finish, and it’s suitable for extreme corrosive environments |
The Standard of Salt Spray Test in Stainless Steel Castings
There is a comprehensive set of internal standards for conducting salt spray testing on stainless steel castings. Knowing these standards can help you gain a technical edge when purchasing stainless steel castings, enabling you to accurately specify the salt spray test standard so that the castings provided better meet your requirements.
ASTM B117
ASTM B117 is the most widely used standard for the neutral salt spray test in the global industrial, automotive, and mechanical industries, and it includes all the parameters required for salt spray testing.
- Salt Solution Concentration
This standard requires the salt solution to be 5% ± 1% NaCl. This is because a solution at this concentration can cause severe corrosion damage to stainless steel castings in the shortest time.
- Chamber Temperature
The chamber temperature must be maintained at 35°C ± 1.1–1.7°C under this standard. This is to increase the corrosion rate by 2–3 times without disrupting the corrosion mechanisms under normal operating conditions.
- Solution pH
The solution pH must be maintained between 6.5 and 7.2 under this standard. This simulates the pH of natural seawater and rainwater, providing the most accurate reflection of the corrosion resistance of stainless steel castings in industrial, marine, and outdoor environments.
Furthermore, a neutral solution can prevent it from directly dissolving the passivation film through chemical reactions, giving a thorough assessment of passivation film integrity on the surface of the stainless steel castings.
- Test Duration
There is no clear distinction regarding the duration of salt spray testing for stainless steel castings; it is primarily determined by your casting’s application and manufacturer’s operating specifications.
However, based on standard industry benchmarks, the widely accepted exposure time for 304 (CF8) is 96 hours. In contrast, for Molybdenum-bearing alloys like 316 (CF8M) and 316L (CF3M), the benchmark is 240 hours or more.
To help you specify the exact requirements in your purchase orders of stainless steel castings, here is a quick-reference summary of industry benchmarks.
| Cast Grade | Common Grade | Industry Benchmark |
| CF8 | 304 | 24-48 hours 96-120 hours after pickling and passivation |
| CF8M | 316 | 96 hours 240-300 hours after pickling and passivation |
| CF3M | 316L | 48–96 h 240-480 hours after pickling and passivation |
| Any Grade | Electropolished Castings | 500-1000+ hours |
Key Applications Need Salt Spray Test
Salt spray testing is important for stainless steel castings, but not all industries require it. Only castings operating in high corrosive environments need to be tested to ensure their corrosion resistance meets the required standards. Understanding which industries require salt spray testing for stainless steel castings will help you determine whether testing is necessary based on your castings’ operating environment and help you reduce the cost of salt spray testing.
Marine Industry
Stainless steel marine castings, such as pump housings and valve bodies, are exposed to seawater with high chloride ions, which can lead to pitting corrosion. If these castings have not undergone rigorous salt spray testing lasting 240–500+ hours, they may develop red rust within a few months, leading to fluid leaks and preventing the pump and valve system from operating normally.
Chemical Industry
Stainless steel castings used in the chemical industry, such as pipe fittings, flanges, and valve body castings, are subject to continuous exposure to highly corrosive media, including acids and alkalis. Only by undergoing 240–300+ hours of salt spray testing can these castings be prevented from intergranular corrosion, which could lead to media leaks or even explosions.
Automotive Industry
Large amounts of sodium chloride are spread on roads to melt snow in winter. If stainless steel components such as automotive undercarriages and exhaust system fittings have not undergone a 96- to 240-hour salt spray test, they will corrode due to sodium chloride spray from the roadside when driving at high speeds.
Food Industry
Stainless steel investment castings in the food industry must maintain absolute purity, so sanitary-grade pumps, valves, and agitator blades made of stainless steel must withstand frequent strong acid and alkali cleaning. Only by passing a 96–240-hour salt spray test can it be ensured that there are no free iron ions on the castings, which could cause red rust contamination of food.
Final Thought
Believe you now know about salt spray testing for stainless steel castings. In highly corrosive operating environments, this testing is essential for helping you avoid unnecessary costs. If you’re looking for a reliable stainless steel investment casting manufacturer, please contact us. Pingheng Machinery, with its standardized salt spray testing procedures, strict quality control, and one-stop customized services, can provide you with high-quality stainless steel castings.
FAQ
Q1. How do you guarantee that every batch of stainless castings passes the salt spray test?
We implement strict quality control measures to guarantee salt spray resistance. Before melting, we use a spectrometer to ensure that the chemical composition of the castings meets specifications. After casting, we perform standard solution heat treatment at 1050°C and transfer the casting to a water-quenching tank as rapidly as possible. During machining, the use of cutting tools on carbon steel is strictly prohibited to prevent contamination by free iron. During post-processing, we use standardized pickling and passivation to ensure a dense, uniform passivation film.
Q2. Why does a ‘rust-resistant’ stainless steel casting still require a salt spray test?
Stainless steel is not completely rust-resistant; its protective film is very thin and prone to contamination in highly corrosive environments. Salt spray testing, which accelerates destructive testing, can identify this risk in advance and ensure that stainless steel castings will not rust during long-term use in a corrosive environment.
Q3. Does 100% chemical composition compliance guarantee passing the salt spray test?
No, chemical composition alone does not guarantee a salt spray pass, which is only the baseline. Use of carbon steel during grinding, improper heat treatment, and improper post-processing can all affect the corrosion resistance of stainless steel castings. Even if the spectral analysis is 100% compliant, if these steps are not done correctly, the stainless steel castings will still rust within a few hours during the salt spray test.
Q4. Is salt spray testing mandatory for all stainless steel castings?
No, ordinary industrial components used indoors or immersed in oil systems do not require salt spray testing. Only castings intended for marine engineering, chemical industry, or other highly corrosive environments are required to undergo testing before leaving the factory.
Q5. What do you do if a casting fails the salt spray test?
We will immediately investigate the cause, first determining whether it is pseudo-rusting by free iron ions on the surface or actual rusting. If it is pseudo-rusting, the casting will need to undergo pickling and passivation again to remove surface impurities; if it is intergranular corrosion, the casting will need to undergo high-temperature solution treatment again. Afterward, we will take samples from this batch of stainless steel castings for salt spray testing and will not ship them until they meet the standard 100%.
