Views: 891 Author: Liu Yanhui Publish Time: 2026-04-10 Origin: Tsingri Screw
There is a specific engineering problem at the heart of every coastal roofing or solar installation project: you need a fastener strong enough to drill through steel, resistant enough to last decades in a corrosive environment, and electrochemically safe to use with aluminium.
No single material satisfies all three requirements. Carbon steel can drill, but it corrodes. Pure stainless steel resists corrosion, but it cannot generate the hardness required to drill through structural steel reliably. And any carbon steel fastener in contact with aluminium will trigger galvanic corrosion — steadily destroying both materials from the point of contact outward.
Bi-metal self-drilling screws solve this problem by combining two materials in a single fastener: a high-tensile steel drill point for mechanical performance, and a stainless steel body for corrosion resistance. This article explains the engineering behind bi-metal construction, when to specify it over single-material alternatives, and what to look for in product performance.
A bi-metal self-drilling screw is a fastener constructed from two distinct metal alloys joined at the drill point:
The body, shank, and thread: manufactured from austenitic stainless steel (SS304 or SS316)
The drill point: manufactured from high-tensile alloy steel (SCM435, a chromium-molybdenum grade)
The junction between the two materials is typically achieved through friction welding or a similar joining process, producing a metallurgically bonded interface that maintains mechanical integrity under installation torque and service loads.
The result is a fastener that behaves like high-tensile carbon steel when drilling — and like stainless steel for the rest of its service life.
Austenitic stainless steels (SS304, SS316) are iron-chromium-nickel alloys with a face-centred cubic crystal structure. This structure gives them excellent ductility and corrosion resistance, but it also limits their hardness. Even with work hardening, austenitic stainless cannot achieve the Rockwell C hardness values required to cut efficiently through mild steel.
In practice, a self-drilling screw tip made entirely from SS304 will:
Generate excessive heat at the drill point through friction
Work-harden the substrate material before cutting through it
Stall or strip before fully penetrating steel above approximately 0.8mm thickness
Produce an irregular hole with torn edges rather than a clean drill cut
This is not a marginal limitation — it is a fundamental material property constraint. SS304 cannot be heat-treated to the hardness levels that carbon and alloy steels achieve through quenching and tempering.
SCM435, by contrast, is a chromium-molybdenum low-alloy steel that achieves Rockwell C hardness values of 45–55 after heat treatment. This is the hardness range required for self-drilling fasteners to cut cleanly and efficiently through structural steel without stalling.
SCM435 (equivalent to SAE 4135, a chromium-molybdenum alloy steel) is selected for bi-metal drill points for three specific reasons:
1. Hardenability
SCM435 achieves high surface hardness with through-hardening capability, maintaining cutting performance at depth rather than only at the very tip. This matters when the fastener is drilling through multi-layer assemblies.
2. Toughness at hardness
Chromium-molybdenum steels combine hardness with toughness — they resist brittle fracture during installation better than high-carbon steels at equivalent hardness levels. A brittle drill point that fractures during installation is not recoverable; the fastener must be extracted and replaced.
3. Weldability at the joint
SCM435 can be reliably joined to SS304/316 through friction welding, producing a joint that does not introduce a stress concentration or corrosion-prone intermetallic zone at the bi-metal interface.
When two dissimilar metals are in electrical contact in the presence of an electrolyte (water, humidity, saline air), a galvanic cell forms. The less noble metal acts as the anode and corrodes preferentially, while the more noble metal acts as the cathode and is protected.
The galvanic series ranks metals by their electrochemical potential. The further apart two metals are in this series, the more aggressive the galvanic corrosion when they are coupled.
Aluminium and mild steel are separated by a meaningful potential difference. When a carbon steel fastener is used to fix aluminium sheeting to a steel substrate:
The carbon steel fastener and the steel substrate are at similar potential — relatively stable
But the aluminium sheet is significantly more active (anodic) than both
In the presence of moisture, aluminium corrodes at the contact zone: the sheet, not the fastener, is sacrificed
In coastal environments — where the electrolyte (saline humidity) is omnipresent — this corrosion is aggressive and progressive. Within a few years, the contact zones around fasteners develop white oxide deposits, then pitting, then structural loss of the aluminium sheet.
The solution is to use a fastener whose surface materials are compatible with aluminium in the galvanic series: stainless steel. SS304 and SS316 are at a much closer electrochemical potential to aluminium than carbon steel, and the galvanic driving force is substantially reduced.
This is why bi-metal screws — with their stainless steel shank and thread in contact with the aluminium sheet — are the correct fastener for aluminium-to-steel fixing. The drill point (SCM435) contacts only the steel substrate below, not the aluminium above.
This is the primary application. Aluminium profiled roofing sheets are fixed to galvanised or painted steel purlins using bi-metal screws. The stainless body contacts the aluminium; the SCM435 point drills through and engages the steel.
Standard specification: double thread design, #12 or #14 gauge, with EPDM or SS304+EPDM bonded washer.
In environments classified AS3566 Class 4 or equivalent — within 1km of coastline, in tidal zones, or in marine industrial areas — carbon steel fasteners with even the best surface coatings have a finite service life that may not match the building's design life.
Bi-metal screws, with their austenitic stainless body, offer corrosion resistance comparable to solid stainless fasteners for the majority of the fastener surface area — while maintaining the drilling performance needed for efficient installation.
Solar installations increasingly use aluminium framing rails and mounting brackets fixed to steel purlins or rooftop structural steel. The bi-metal screw is the preferred fastener for aluminium rail-to-steel connections because:
System warranties often extend 25–30 years: fastener lifespan must match
Coastal and rooftop environments are classified as corrosive
Galvanic compatibility between fastener and aluminium rail is a warranty requirement for many system manufacturers
Aluminium composite panel (ACP) systems fixed to steel framing require fasteners that will not corrode in contact with the aluminium skin. Bi-metal screws with countersunk or pan heads maintain the visual integrity of the cladding face while providing compatible galvanic properties.
Bi-metal thin sheet self-drilling screws are available in two thread configurations, each optimised for a different substrate combination.
The single thread design is optimised for fixing thin gauge metal sheets together — for example, fixing two layers of 0.75mm aluminium composite facing, or fixing 1.0mm sheet to a light frame.
Characteristics:
Coarser thread pitch for softer materials
Optimised drill point for minimal deformation in thin sheet
Lower installation torque
Not designed for fixing to structural steel
The double thread design is the bi-metal screw for steel substrate applications. The double helix thread provides two advantages simultaneously:
Faster thread engagement: Two thread starts means the screw advances twice as fast per revolution compared to a single thread, reducing installation time and heat generation.
Dual material compatibility: The double thread performs in both the aluminium sheet (above) and the steel substrate (below), creating a strong connection in both materials from a single fastener pass.
Maximum steel drilling capacity: Up to 20mm steel plate in a single operation, without pre-drilling.
For most roofing and solar panel applications — aluminium-to-steel connections — the double thread bi-metal screw is the correct specification.
Applications: thin sheet-to-thin sheet, 1×1.0mm or 2×0.75mm substrate limit.
Applications: aluminium-to-steel, solar mounting, coastal roofing.
All bi-metal screws are available with hex washer head + EPDM washer as standard, with SS304+EPDM bonded washer available as an upgrade for Class 4 environments.
The bi-metal body being SS304/316 stainless provides the primary corrosion resistance. However, the SCM435 drill point — as a carbon alloy steel — requires surface protection to prevent rust initiation at the tip before and during installation.
Standard treatment applied to the drill point includes a chromate or phosphate conversion coating, which provides short-term protection during storage and shipping. Once installed, the drill point is inside the steel substrate, significantly reducing its exposure to corrosive environments.
For the highest corrosion performance, SS316 body construction (rather than SS304) should be specified in:
Marine environments (tidal zones, saltwater splash)
Chemical processing environments
Projects with 30+ year design life requirements
SS316 contains 2–3% molybdenum, which increases pitting resistance in chloride environments — the specific corrosion mechanism dominant in coastal and marine exposure.
The cost premium of bi-metal over coated carbon steel is typically 30–50% per fastener. Against a fastener replacement cycle of 10–15 years for carbon steel in coastal environments — including the cost of roof access, membrane disturbance, and warranty implications — bi-metal screws deliver a lower total cost of ownership for long-life projects.
Choose bi-metal self-drilling screws when any of the following conditions apply:
The substrate includes aluminium sheeting or panels in contact with the fastener shank
The project environment is classified coastal (within 1km of coastline) or marine
The system design life exceeds 20 years
The installation involves solar panel mounting rails
The project warranty requires electrochemical compatibility documentation between fastener and substrate
Choose double thread for aluminium-to-steel connections and structural steel substrates. Choose single thread for thin sheet-to-thin sheet applications where the substrate is not structural steel.
Bi-metal self-drilling screws are not a premium option for premium projects. They are the technically correct fastener for a specific and increasingly common set of application conditions: aluminium in contact with steel, corrosive environments, and long-design-life building systems.
The engineering behind the SS304+SCM435 combination is precise: it delivers what neither material achieves alone — the ability to drill through steel, and the ability to resist corrosion and galvanic reaction for decades of service.
For project-specific technical support or bi-metal screw enquiries, contact info@tsingri.com.
