1. SPA-H Weathering Steel at a Glance
SPA-H is defined in JIS G3125: Hot-rolled atmospheric corrosion resisting steels for traffic, containers and similar applications. It is the most widely specified grade of the SPA family and is characterised by an alloying system of copper, chromium and phosphorus that gives it substantially better atmospheric corrosion resistance than plain carbon steel. SPA-H is routinely produced as hot-rolled sheet and plate in thicknesses of about 1.6 mm to 25 mm, which suits the thin sections used in container corner posts, rail car bodies and solar mounting profiles.
2. Chemical Composition and Mechanical Properties (JIS G3125)
Representative composition limits for SPA-H per JIS G3125 are shown below.
| Element | Limit (max %, unless shown as a range) |
|---|---|
| C | 0.12 |
| Si | 0.20–0.75 |
| Mn | 0.20–0.60 |
| P | 0.070–0.150 |
| S | 0.035 |
| Cu | 0.25–0.55 |
| Cr | 0.30–1.25 |
| Ni | 0.65 |
In the hot-rolled condition, SPA-H sheet and plate typically provides a minimum yield strength of 355 MPa for thicknesses up to 16 mm, a tensile range of 490–610 MPa and a minimum elongation of 15%, in line with the JIS G3125 property tables.
3. How the Patina Works in Solar Environments
The corrosion resistance of SPA-H comes from a dense, compact oxide layer that develops on the surface when the steel is exposed to alternating wet and dry cycles. The main protective constituent is a fine-grained goethite-type rust (alpha-FeOOH) that restricts oxygen and moisture access to the metal surface. For the mechanism to function, the surface must be allowed to dry out regularly; locations that remain permanently damp, are buried, or receive continuous chloride splash cannot develop a protective patina and will corrode like ordinary steel. In a typical inland solar site with moderate rainfall, the patina stabilises within one to three years, after which the corrosion rate falls to a low level that can be accommodated with an appropriate thickness allowance.
4. Design Considerations for Solar Panel Mounting Structures
Solar mounting systems impose specific requirements on the steel. First, sections are often thin (1.5–6 mm), so the atmospheric corrosion allowance should be added to the structural thickness rather than compensated by over-designing joints. Second, bolted connections are preferred over welded ones for field assembly, and the fasteners should be galvanized or stainless steel to avoid accelerated corrosion at the interface. Third, water should be allowed to drain freely; profiles that trap water, collect debris or shield parts of the structure from drying should be avoided. Fourth, direct galvanic contact between weathering steel and aluminium or zinc-coated components in the presence of water should be prevented with isolating washers or paint. Finally, sites within about 1 km of salt water, or with heavy de-icing salt exposure, are generally outside the economical range of bare weathering steel and require a coating system.
5. Maintenance and Service Life Expectations
A correctly detailed SPA-H mounting structure needs no periodic painting, which reduces operating cost and eliminates coating waste during decommissioning. Periodic inspection should focus on drainage points, fastener condition and any areas where run-off from other materials, such as copper flashings, could discolour or attack the steel. Stainless steel and aluminium components in the same structure should be separated from the weathering steel by non-absorbent isolating material where they are in contact.
Frequently Asked Questions
What does SPA-H mean in JIS G3125? SPA identifies the atmospheric corrosion resisting steel family of JIS G3125; the H suffix denotes the higher-strength grade of the SPA series, with minimum yield of 355 MPa at 16 mm and below and tensile strength of 490–610 MPa.
Is SPA-H suitable for coastal solar farms? Bare SPA-H is generally not recommended where structures are continuously exposed to sea salt aerosols. In coastal zones the patina cannot stabilise, so the steel corrodes at a much higher rate; coated weathering steel or alternative materials should be considered within roughly 1 km of the coastline.
Does the patina stain other materials? Yes. During the first one to three years, run-off water may carry rust-coloured streaks onto concrete foundations, paving or light-coloured surfaces. Drainage design and, where necessary, a temporary runoff barrier mitigate this.
How long does a solar mounting structure in SPA-H last? With correct detailing and a suitable atmospheric environment, the structure can remain serviceable beyond the 25–30 year PV project lifecycle, provided the section thickness includes the expected corrosion allowance and inspections are carried out at regular intervals.
Can SPA-H be welded for racking components? Yes. SPA-H welds readily with standard arc processes using low-hydrogen or weathering-steel-compatible consumables. Where racks are welded, the welds should be ground smooth and, if left uncoated, allowed to patinate like the parent metal.
Is galvanizing compatible with SPA-H? Galvanized fasteners and brackets can be used, but direct water-exposed contact between bare SPA-H and zinc should be isolated, because run-off from weathering steel can accelerate zinc corrosion. Isolating washers or sealing compounds are the standard remedy.
