The Salt Spray Test (ASTM B117) Explained: How We Measure Corrosion Resistance

When you request an electroplated finish for your components, the primary goal is almost always to prevent corrosion. But how do you quantify “corrosion resistance”? You can’t wait five years for a part to rust in the field to determine if your plating vendor did a good job.

This is where accelerated corrosion testing comes in, and the undisputed king of these tests is the Salt Spray Test, formally standardized as ASTM B117.

In this article, we break down how the test works, what the results mean for your engineering specifications, and why it remains a cornerstone of quality control at Platinex Industries.

What is ASTM B117?

ASTM B117 (Standard Practice for Operating Salt Spray/Fog Apparatus) is not a test that dictates a specific pass or fail metric. Instead, it is a strictly controlled set of environmental conditions designed to accelerate the corrosion process.

It provides a standardized, repeatable environment so that manufacturers can compare the relative corrosion resistance of different coatings or verify that a specific batch of plated parts meets historical performance baselines.

How the Salt Spray Test Works

The testing process is surprisingly straightforward but requires meticulous control of several variables:

1. The Chamber: Parts are suspended inside an enclosed testing chamber. They must be angled (usually 15 to 30 degrees from vertical) so that the corrosive solution doesn’t pool on flat surfaces but rather runs down.
2. The Solution: A 5% sodium chloride (NaCl) solution is prepared using highly purified water.
3. The Environment: The chamber is heated and maintained at a constant 95°F (35°C).
4. The Fog: The saline solution is atomized into a continuous, dense fog that settles uniformly over the parts.

The parts remain in this continuous salt fog 24 hours a day. They are periodically inspected at set intervals (e.g., 24, 48, 96, 120, 240 hours) until signs of corrosion appear.

Decoding the Results: White Rust vs. Red Rust

When evaluating zinc-plated components, inspectors are looking for two distinct milestones during the test:

1. Hours to White Rust

White rust (zinc oxide) is the first sign of corrosion. It appears as a white, powdery residue on the surface of the part.

What it means: White rust indicates that the passivate layer (the clear, yellow, or black chromate coating applied over the zinc) has failed, and the zinc plating itself is now actively corroding to protect the steel underneath.

• Typical Standard: A high-quality trivalent clear zinc finish should withstand at least 96 to 120 hours before white rust appears.

2. Hours to Red Rust

Red rust (iron oxide) is exactly what it sounds like: the familiar red/brown rust that indicates the steel substrate is corroding.

What it means: Red rust indicates a complete failure of the coating system. The passivate has failed, the sacrificial zinc layer has been completely consumed in that area, and the base metal is now under attack.

• Typical Standard: Depending on the thickness of the zinc layer (microns), a good plating job should survive anywhere from 240 to over 400 hours before red rust appears.

Limitations of the Salt Spray Test

While ASTM B117 is the industry standard, engineers must understand its limitations:

• It Does Not Predict Real-World Lifespan: 100 hours in a salt spray chamber does not equate to X number of years outdoors. The test provides a continuous, wet, highly saline environment. Real-world environments fluctuate with dry cycles, UV exposure, and different pollutants (like sulfur dioxide).
• It Can Disadvantage Certain Coatings: The salt spray test is notoriously harsh on porous coatings and heavily favors coatings that excel in continuously wet, high-chloride environments. For example, hot-dip galvanizing performs brilliantly outdoors for decades but can fail surprisingly fast in a salt spray chamber compared to advanced zinc-nickel alloys.

Why We Rely on It at Platinex

Despite its limitations, ASTM B117 remains invaluable for one primary reason: Consistency and Quality Control.

If we establish that a specific 10 µm zinc plating process with a trivalent clear passivate yields 120 hours to white rust during our initial engineering qualification, we can use the salt spray test to verify that our daily production maintains that exact standard.

At Platinex Industries, we utilize continuous salt spray testing to audit our chemistry baths, validate new sealing technologies, and guarantee that the components we ship to OEMs in the automotive and electrical sectors will perform precisely as specified.

Need a vendor that understands the science behind the specification? Contact our quality control team today to discuss your testing requirements.