Designing and testing effective rust preventives for metalworking

For industries reliant on metal components, rust is more than a nuisance—it can mean the difference between product longevity and costly failures. Whether a manufacturer is shipping precision-engineered automotive parts or a home improvement store is stocking screws, rust preventives are critical to protecting metal surfaces from corrosion.

Metalworking professionals depend on high-performance rust preventive fluids to safeguard finished metal products from environmental damage, ensuring product quality and preventing reputational risks. The right rust preventive solution must not only offer effective corrosion protection but also align with environmental regulations and operational efficiencies.

With these challenges in mind, this article explores key performance characteristics of rust preventives, alternative additive options, and testing methodologies to ensure optimal protection.

Key characteristics of an effective rust preventive

A well-formulated rust preventive must deliver multiple performance benefits to withstand varied industrial conditions. The most essential include:

• Corrosion Protection: The primary function of a rust preventive is to resist rust formation, ensuring metal remains intact when exposed to fluctuating temperature, humidity, and environmental conditions.
• Water Displacement: During manufacturing, metal parts often retain moisture from previous processing steps. A rust preventive must effectively displace water from the metal surface to prevent trapped moisture, which can accelerate corrosion.
• Water Separation: Once a wet metal part is treated, the displaced water should separate from the rust preventive and settle at the bottom of the tank. Efficient water separation allows operators to drain excess water without changing fluids frequently, improving operational efficiency.
• Removability: Certain applications, such as pre-painting processes, require rust preventives that can be easily removed without leaving residues that could interfere with coatings or adhesives.
• Lubricity: In some cases, rust preventives must provide additional lubricity to facilitate manufacturing processes or further processing steps.

Exploring additive options

Achieving these critical performance attributes depends on the right additive chemistry. Rust preventive formulators have several key options:

• Barium Sulfonate: Traditionally used in solvent-based rust preventives, barium sulfonate offers strong corrosion protection and excellent water separation. However, increasing regulatory scrutiny—particularly from the U.S. Clean Water Act and European REACH regulations—has prompted a search for safer alternatives.
• Calcium Sulfonate: A widely accepted alternative to barium sulfonate, calcium sulfonate provides comparable corrosion resistance and water separation while maintaining a more favourable environmental profile.
• Sodium Sulfonate: Commonly used in water-based rust preventives, sodium sulfonate acts as an emulsifier, making it suitable for applications requiring high compatibility with aqueous solutions.
• Film-Formers: These additives work synergistically with sulfonates to create a durable protective barrier on metal surfaces. Film-formers derived from petroleum or natural waxes enhance corrosion resistance and enable self-healing properties if the protective layer is damaged.

While barium sulfonate has historically been favoured for its superior performance, modern calcium sulfonates have proven to match or exceed its water displacement and separation capabilities. Given increasing regulatory pressure, transitioning to calcium sulfonate-based solutions presents a viable path forward for formulators.

Validating rust preventive performance through testing

To ensure rust preventives perform effectively in real-world conditions, rigorous testing is essential. Unlike other industrial fluids, rust preventives are not subject to universal specifications due to their wide-ranging applications. Instead, formulators must conduct targeted accelerated laboratory testing to validate performance. Key testing methodologies include:

• Salt Spray Testing (ASTM B117-19): This accelerated corrosion test exposes metal panels to a 5% sodium chloride mist at 35°C (95°F) until visible rust forms. It is commonly used for evaluating thin-film and heavy-duty rust preventives in outdoor storage conditions.
• Humidity Cabinet Testing (ASTM D1748-22): Designed for indoor storage applications, this test subjects metal panels to 100% humidity at 49°C (120°F) to assess rust preventive longevity. Effective formulations typically provide protection for 30 to 60 days under these conditions.
• Water Displacement Testing: A critical evaluation method where metal panels or steel wool samples are submerged in a rust preventive solution after being saturated with water. The ability to completely remove moisture from the surface determines the fluid’s effectiveness in preventing trapped water corrosion.
• Water Separation Testing: This test measures how effectively a rust preventive can separate water carryover in a rinsing process. The formulation is mixed with water, and the time required for clear separation is recorded. Faster separation indicates higher efficiency.

Since rust preventive testing requires specialised equipment, many formulators lack in-house testing capabilities. Working with an additive supplier that offers internal testing services can streamline the development process and ensure formulations meet performance requirements.

Conclusion: The future of rust preventives

As metalworking industries continue to prioritise corrosion protection, regulatory compliance, and operational efficiency, rust preventives must evolve. Formulators are increasingly shifting away from barium-based solutions toward environmentally responsible alternatives such as calcium sulfonate-based formulations.

Additionally, advances in testing protocols are allowing manufacturers to validate performance more accurately, ensuring that rust preventives can withstand real-world environmental conditions. By leveraging innovative chemistry and partnering with expert additive suppliers, formulators can continue to deliver high-performance rust protection that meets the demands of modern industry.

About the authors

Amelia Hadler, Ph.D. is a technology development manager in the Metalworking Additives Group at Lubrizol. She has 11 years of experience in metalworking fluid development, focusing on rust preventives and corrosion inhibitors.

Maria Shepherd, B.S. is a research chemist at Lubrizol with expertise in metalworking technical service and fluid testing. She has extensive experience evaluating rust preventives, greases, and emulsions for real-world durability.