What Is a Salt Spray Test for Rubber Parts?

A rubber part can pass dimensional checks and still fail outside. Salt mist can attack metal inserts, coatings, and bonds, and then the whole assembly starts leaking or loosening.

A salt spray test exposes rubber parts (often with metal inserts, clamps, or coatings) to a controlled salt fog to evaluate corrosion risk and related failures like bond damage, coating breakdown, and sealing loss.

I usually explain salt spray testing in one sentence: rubber does not rust, but rubber assemblies often fail because the metal and interfaces around the rubber start to rust or break down. That is why I treat this test as an “assembly reliability” test, not a “rubber-only” test.

If Rubber Does Not Rust, Why Do I Run a Salt Spray Test?

Salt spray testing sounds like a metal topic, so many buyers ask me why it appears on a rubber RFQ. I see a clear reason in real projects.

I run salt spray tests for rubber parts when the rubber is part of an assembly that includes metal, coatings, spring clips, or rubber-to-metal bonding, because corrosion can destroy sealing performance even if the rubber itself stays intact.

What salt spray really targets in rubber projects

I usually see salt spray requirements in these product types:

• ✅ Rubber gaskets with steel backing¹ or metal carriers
• ✅ Molded rubber parts with brass or steel inserts²
• ✅ Rubber seals used with hose clamps, springs, and fasteners³
• ✅ Rubber-to-metal bonded parts like mounts and bushings
• ✅ Assemblies with plated or painted⁴ metal surfaces near the sealing line

In these cases, corrosion can create sharp edges, lifting, flaking, and loss of clamp load. Then sealing stress drops. Then micro-leaks start. I have seen projects where the rubber stayed flexible, but the insert rust expanded and cracked the rubber edge. I have also seen plating blister and peel, and the bond line failed after that.

How I frame the business risk

Salt spray is often used as an accelerated screen. It is not a perfect weather simulator. ASTM B117⁵ itself states it covers the apparatus and procedure and does not prescribe exposure periods or how results must be interpreted. I treat this statement as the key warning for buyers. The test is useful, but only when the acceptance criteria⁶ are defined clearly.

A simple “when I recommend it” table

When I receive a salt spray requirement, I ask one question first: “Which failure matters most: rust grade, bond strength, or sealing function?” That question decides the test plan.

Which Salt Spray Standards Are Common, and What Do They Actually Measure?

Many purchase orders list “salt spray 240 h” without a standard name. That creates confusion and disputes. I avoid that by aligning on a named standard and a clear evaluation method.

The most common salt spray standards are ASTM B117 and ISO 9227, and they mainly assess corrosion resistance of metals and coatings under controlled salt fog conditions. For rubber projects, I use these standards to evaluate the metal and interfaces in the rubber assembly.

The two names I see most in RFQs

• ASTM B117: a widely used practice for operating a salt spray (fog) apparatus. It focuses on how to run the environment and does not define product-specific pass criteria by itself.

• ISO 9227⁷:2017: specifies procedures for neutral salt spray (NSS), acetic acid salt spray (AASS), and copper-accelerated acetic acid salt spray (CASS) for assessing corrosion⁸ resistance of metallic materials with or without protection.

I treat ISO 9227 as very useful when a buyer wants NSS vs AASS vs CASS spelled out clearly. I also treat ASTM B117 as very common when buyers are referencing automotive and coatings supply chains.

How I choose between NSS, AASS, and CASS⁹

I do not pick the harshest method by default. I pick the method that matches the plating system and the real risk.

ISO 9227 explicitly covers NSS, AASS, and CASS methods. I also see ASTM G85 used when buyers want modified or cyclic salt fog options beyond classic B117-type conditions. ASTM G85 describes multiple modified salt spray approaches for specification purposes.

A practical rule I follow

I never accept “salt spray hours” alone as a complete requirement. I always align these items:

• standard name and method (ASTM B117 / ISO 9227 NSS / others)
• exposure duration (hours)
• evaluation method (rust grade, blistering, adhesion, functional sealing)
• test sample definition (with or without clamps, assembled or not)

This step is where I prevent most misunderstandings.

How Do I Specify a Salt Spray Test for Rubber Parts in a Purchase Order?

When I write a tight RFQ, I aim for a test that protects the buyer and is fair to the supplier. Salt spray can become unfair when the specimen definition¹¹ is unclear.

I specify salt spray testing for rubber parts by defining the standard, exposure time, test configuration (assembled or not), metal finish requirements, and acceptance criteria like maximum rust area, coating condition, and functional checks such as torque retention or leak testing.

The four decisions that control the outcome

I treat these as non-negotiable:

1) What is the specimen: rubber only, rubber + insert, or full assembly with clamp
2) What is the finish: stainless, zinc plated, e-coat, powder coat, passivated, or painted
3) What is the evaluation: cosmetic grade, corrosion grade, or functional grade
4) What is the acceptance: rust limits, coating limits, and post-test function checks

ASTM B117 itself warns that it does not define exposure periods or interpretation for a specific product. That is why I write acceptance rules in the PO.

What I usually put into a buyer-friendly clause

I use simple language like this and I adjust it per project:

• Standard: ISO 9227 NSS (or ASTM B117)
• Duration: 96 h / 240 h / 480 h (as agreed)
• Specimen: “as assembled,” including clamps and fasteners, or “insert only,” as agreed
• Orientation: define if the sealing surface must face up, or if free-hanging is allowed
• Acceptance: maximum red rust %, no coating blistering beyond defined level, no bond lift, and no sealing loss after re-assembly

A structured PO table I recommend

My factory-side habit that helps buyers

I always request photos of the test rack setup and specimen orientation, because orientation can change how salt accumulates and drains. I also ask for pre-test photos of the metal surface condition. This is not about arguing. This is about making the result repeatable and comparable.

What Failures Should I Check After Salt Spray Exposure?

Some projects treat salt spray as a “rust only” check. I treat it as an interface and function check, because those failures create the real cost.

After salt spray exposure, I check not only rust and coating damage, but also rubber-to-metal bond integrity, clamp load retention, insert expansion effects, sealing performance, and any cracking or swelling that could change fit.

What I inspect in a consistent order

I use a fixed sequence so results are comparable:

• ✅ Visual grading: red rust, white rust, blistering, underfilm corrosion
• ✅ Edge and interface check: insert edges, cut lines, coating lift near the rubber
• ✅ Bond line check: peel, lift, or creeping corrosion at rubber-to-metal interface
• ✅ Function check: re-assembly torque feel, clamp retention, or leak test if relevant
• ✅ Dimensional check: swelling, distortion, insert-driven cracks

ISO 9227 is written for assessing corrosion resistance of metallic materials and their protective systems. In rubber projects, that means the “pass” often depends on what happens to the metal that supports sealing and fastening.

The failure modes I see most in rubber assemblies

Where material selection still matters

Even though rubber does not “rust,” rubber selection still affects salt-environment durability:

• Salt + heat can accelerate aging for some compounds
• Salt water exposure can carry contaminants, cleaners, and oils
• Some applications combine salt fog with ozone, UV, and cyclic drying

I treat chemical compatibility¹² as a wider question than salt alone. I also treat hardness as a stability decision, because a soft rubber can lose contact stress faster when clamp load relaxes. I treat temperature as critical because higher temperature often accelerates aging and compression set. I treat certifications as a constraint because certain compliance compounds can limit additive packages.

A quick performance comparison I share with buyers

In real projects, I pair this rubber selection table with a metal finish decision. The best rubber can still fail if the insert and plating are weak for the environment.

Conclusion

The salt spray test is an indispensable quality gate for any rubber part containing metal, proving resistance to corrosion and the integrity of the rubber-to-metal bond.