Why coatings and stainless choices matter for license plate holders
Corrosion starts quietly and accelerates with winter salt, moisture and temperature swings. In SleemPlate’s ecosystem we use magnets with a Zn plus black epoxy finish and screw mounting. The coating and screw material affect not only the look but also real-world durability in wet, salty conditions.
Why magnets need protection
Neodymium magnets are corrosion-prone unless protected. A proper coating forms a barrier against moisture and chemicals, helping preserve performance. Multiple technical sources highlight epoxy’s strong barrier properties and its suitability for harsh, salt-exposed environments.
The logic of Zn + epoxy
Zinc acts as a sacrificial anode. If a pinhole or scratch occurs, the zinc corrodes preferentially and helps protect the area around the defect. Epoxy adds a robust chemical and mechanical barrier, with a premium black appearance. Industrial specs describe this composite approach and the sacrificial role of zinc.
How corrosion resistance is assessed
Industry often uses salt-spray tests per ISO 9227 to compare coatings under accelerated exposure. The test does not mirror every real condition, but it helps reveal coating defects and offers a comparable yardstick across products. ISO material and technical explainers outline the method and limitations.
Stainless screws: A2 vs A4 in the real world
For general automotive use, A2, or 304, is a dependable choice. In salt-rich or chloride-exposed environments, A4, or 316, benefits from molybdenum, improving resistance to pitting and crevice corrosion. Recent engineering guides repeatedly note this advantage.
In short
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A2: excellent in standard environments.
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A4: preferred when winter road salt or coastal exposure is routine.
Passivation for stainless
Passivation removes free iron from the surface and reinforces the chromium-rich passive layer. The ASTM A967 standard details nitric and citric processes and acceptance tests. In manufacturing, passivation can noticeably improve corrosion resistance, especially after machining or forming.
Galvanic concerns in salty environments
When dissimilar metals are electrically connected in an electrolyte, the less noble metal corrodes faster. Galvanic series resources show how stainless, aluminum, zinc and steel rank in seawater. In vehicles, minimizing unnecessary metal-to-metal bridges and maintaining protective layers reduce galvanic risk.
Thread galling with stainless
Stainless fasteners can gall during tightening, effectively seizing due to adhesive wear. Technical guidance recommends suitable anti-seize products and controlled tightening. In corrosive environments, metal-free anti-seize is preferred to avoid adding galvanic couples.
What this means for a magnet-and-screw system
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Zn + epoxy magnets: chemical barrier plus sacrificial protection at minor defects, ideal for wet and salty exposure.
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Stainless screws: A2 for most use cases, A4 for heavy salt exposure. If you drive through salted roads often, A4 is a sensible upgrade.
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Passivation: valuable for stainless parts after fabrication to bolster corrosion resistance.
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Galvanic awareness: avoid unnecessary mixed-metal contacts in wet areas and keep protective finishes intact.
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Galling control: use appropriate anti-seize, tighten with care.
Common buyer questions
Can epoxy chip?
Any organic coating can chip if edges take a hard hit. The advantage of pairing with Zn is that zinc provides sacrificial protection around small defects. Periodic visual checks are sufficient.
Is A4 “immune” to salt?
No. It is more resistant than A2 thanks to molybdenum, yet pitting and crevice corrosion can still occur in severe conditions. Seasonal cleaning and avoiding deposits under the screw head help.
What do salt-spray hours tell me as a customer?
They are a comparison tool, not a guarantee of lifetime. Salt-spray testing helps reveal coating integrity under accelerated conditions.
Is passivation mandatory for every fastener?
Not always. For high-demand applications, ASTM A967 passivation is a mature industrial practice that improves corrosion performance.
Bottom line
For long-term durability, pairing Zn + epoxy-coated magnets with stainless screws chosen for your environment is the smart path. In heavy winter salt, A4 is worth it. Paying attention to galvanic contacts and galling prevents headaches later. In the SleemPlate ecosystem, all current kits are screw-mounted, no adhesives, include cross-head screws and IPA wipes for surface prep. For replacements or tweaks see Spare Parts or SleemFix. For global plate formats check MagSleem X. For an ultra-slim fixed look see SuperSleem. For quick magnetic on-off choose MagSleem.