7,296-Hour Salt Spray Test: Solaracks Thermal Diffusion Galvanized Screws Exceed ISO CX Corrosion Standard

Published: May 2026 | Test ongoing since July 11, 2025 | Standard: GB/T 10125 Neutral Salt Spray (NSS)

When Solaracks specifies a fastener, we need to prove it, not claim it.

In July 2025, we placed a batch of our thermal diffusion galvanized self-drilling screws (ST6.3×80mm) into a neutral salt spray chamber and set the clock running. Ten months later, as of May 11, 2026, those screws have accumulated 7,296 continuous hours of NSS exposure under GB/T 10125, with structural integrity fully intact and only minor surface rust on some samples.

This post shares the full test progression, explains what the results mean against the ISO corrosion category scale, and clarifies why the surface rust observed at this stage does not represent structural failure for solar mounting applications.

What We Tested

• Product: Self-tapping self-drilling screw, ST6.3×80mm
• Coating: Thermal diffusion galvanizing (热渗锌 / Sherardizing)
• Test standard: GB/T 10125 Corrosion Tests in Artificial Atmospheres — Salt Spray Tests (equivalent to ISO 9227)
• Test type: Neutral Salt Spray (NSS)
• Test start date: July 11, 2025
• Cumulative hours as of May 11, 2026: 7,296 hours (test ongoing)

What Is Thermal Diffusion Galvanizing?

Thermal diffusion galvanizing (also called sherardizing or 热渗锌) is a dry zinc diffusion process where zinc powder is tumbled with the steel fasteners at elevated temperature (approximately 320–500°C). The zinc atoms diffuse into the surface of the steel, forming a dense zinc-iron alloy layer that is metallurgically bonded to the substrate.

This is fundamentally different from conventional electroplating, where zinc is deposited onto the steel surface as a separate layer. The bonded alloy structure of thermal diffusion galvanizing offers:

• Uniform coating thickness, even on thread roots, tips, and complex geometries — the areas most prone to early corrosion on conventional plated screws
• No hydrogen embrittlement risk — critical for high-tensile steel fasteners
• Superior adhesion — the coating cannot delaminate or peel
• High base corrosion resistance — starting from a much thicker effective zinc equivalent

For solar mounting fasteners, which must perform across 25-year project lifetimes in coastal, tropical, industrial, and high-altitude environments, this process offers a significant durability advantage over standard electrogalvanized or phosphate-and-oil finishes.

The Test Progression

Below are the inspection milestones recorded during the ongoing NSS test.

1,152 Hours — No Change

At 1,152 hours, the screws showed no visible red rust, no white rust, and no coating degradation. The thermal diffusion galvanized surface remained dark grey and uniform across heads, threads, and drill points. Standard zinc electroplated fasteners (Grade 3 per ISO 4042) typically show red rust onset at 96 hours. These samples at 1,152 hours were entirely undisturbed.

1,800 Hours — Zinc Sacrifice Begins, No Red Rust

At 1,800 hours, samples showed minor white crystalline deposits on thread recesses. This is zinc corrosion product (zinc oxide and zinc hydroxide), the expected result of the zinc layer sacrificially protecting the steel substrate. No red rust (iron oxide) was present. This is exactly the mechanism thermal diffusion galvanizing is designed to exploit: the zinc corrodes slowly so the steel does not.

3,528 Hours — Coating Fully Intact

At 3,528 hours (147 days of continuous salt spray), the fasteners remained in excellent condition. The dark grey coating was intact across all surfaces. No red rust. Threads remained sharp and well-defined.

4,392 Hours — Still No Structural Degradation

At 4,392 hours (183 days), the protective coating continued to perform. Inspection showed no red rust, no pitting, and no base metal exposure on any of the five samples in this batch.

7,296 Hours — Slight Surface Rust, Structure Intact

At 7,296 hours (304 days), the test reached its latest recorded milestone. The screws remain structurally sound. Three to four samples displayed a light brownish-orange tint in the deepest thread recesses — this is surface rust (浮锈), also called bloom rust. Thread profiles remain sharp, drill points show no blunting, and no base metal pitting is visible.

This is the finding we want to address directly and in full.

What the ISO Scale Says

ISO 9223 classifies atmospheric corrosivity into six categories based on environmental aggressiveness:

CX is the highest classification, the standard against which products destined for offshore platforms, tropical coastal power plants, and severe industrial zones must be measured.

In corrosion qualification testing, products are typically expected to withstand a defined number of NSS hours without significant red rust to be considered fit for a given category. The 7,296-hour result demonstrated by these thermal diffusion galvanized screws surpasses the CX threshold, with only minor cosmetic bloom rust at the outermost surface and no base metal corrosion.

For solar EPCs specifying fasteners for coastal Philippines, humid Vietnam, industrial zones in Malaysia, or the salt-laden air of the UAE and Saudi Arabia coastlines, this result provides a directly relevant performance benchmark.

The Critical Engineering Point: Surface Rust Does Not Mean Mechanical Failure

This is the section that matters most for structural engineers and EPC procurement teams.

Surface rust (浮锈) on zinc-coated fasteners is a cosmetic phenomenon, not a structural failure signal.

Here is what is actually happening at the 7,296-hour stage:

1. The zinc-iron alloy layer is still present. The thermal diffusion galvanized coating forms a zinc-iron intermetallic layer that is metallurgically fused to the steel. This is not a simple zinc overlay. It does not peel, crack, or delaminate.
2. The brownish tint is iron oxide on the zinc surface, not on the steel substrate. Trace iron particles from the manufacturing environment, or the outermost micro-layer of the zinc matrix, can oxidize at extreme NSS durations and produce a faint rust appearance. This is a surface phenomenon confined to microns of depth.
3. Cathodic protection is still active. As long as the zinc layer is present, it continues to act as a sacrificial anode. The zinc corrodes electrochemically to protect the steel core. The steel is not oxidizing.
4. Mechanical properties are determined by the steel core. Tensile strength, torsional strength (installation torque), proof load, and pull-out force are all functions of the steel substrate, not the surface coating. These values are unchanged at the 7,296-hour stage. The screw still pulls down to design clamping force. It still resists the wind and snow loads your racking structure was designed for.

For solar mounting, this distinction is decisive.

A solar mounting structure is a long-term engineered asset. Its fasteners must resist loosening under cyclic wind load, must not lose clamping force due to corrosion-induced section loss, and must not fracture under maintenance loads. Surface bloom rust on a thermal diffusion galvanized fastener, even after over 300 days of continuous coastal-equivalent salt spray, tells us nothing negative about any of these properties.

A screw that shows 浮锈 after 7,296 hours of NSS exposure has demonstrably outperformed most of the planet’s fastener supply. The zinc layer is doing its job.

What This Means for Your Project

If your solar installation is located in:

• Coastal zones (within 1 km of seawater)
• Tropical high-humidity environments (Philippines, Indonesia, Vietnam, Bangladesh)
• Industrial zones with airborne chemical pollutants
• Salt lake or desert regions with high salinity dust
• C4 or C5+ corrosivity classification sites

…then fastener corrosion resistance is not a minor detail. It is a 25-year structural risk factor.

The 7,296-hour NSS result on Solaracks thermal diffusion galvanized screws demonstrates a product rated beyond CX class, the harshest ISO category defined for atmospheric corrosion, while maintaining full structural functionality. Surface rust at this duration is expected and immaterial to mechanical performance.

We will continue to update this test log as the trial runs past 8,000, 10,000, and 12,000 hours.

About This Test

All testing was conducted under GB/T 10125 Corrosion Tests in Artificial Atmospheres — Salt Spray Tests at a certified laboratory. NSS conditions: 5% NaCl solution, pH 6.5–7.2, chamber temperature 35°C ± 2°C, continuous spray. Test start date: July 11, 2025. The trial is ongoing.

Product: ST6.3×80mm self-tapping self-drilling screw, thermal diffusion galvanized (热渗锌), tested per standard sample batch protocol.

Work With a Team That Proves It

At Solaracks, corrosion resistance is not a specification we write on a datasheet and leave unverified. It is a commitment we put in a test chamber and run for years.

If your next project involves a corrosive environment and you need mounting hardware that will hold for 25 years, we will supply the engineering data to back it up.

Quote and technical documentation within 24 hours.
Contact Solaracks