Replacement

Brass vs. Aluminum, Steel, and Plastic: Comparing Mail Slot Materials

The material a mail slot is made from affects how it resists corrosion, responds to wear, and ages outdoors. While the sealing performance of a finished product depends on its overall design, the underlying material still influences long-term durability and appearance. Here's how the most common materials compare.

Hardness and Wear

Compared with carbon steel, brass is generally softer but offers different advantages, including natural corrosion resistance and excellent machinability. Its Brinell hardness runs around 60 for soft brass depending on the specific alloy, compared to 130 or more for mild steel, though brass is harder than aluminum.¹ Softer doesn't mean weaker in every sense that matters for hardware: brass is commonly chosen because it forms and holds its shape well while still standing up to years of repeated handling.

Corrosion Behavior

Ordinary carbon steel needs an intact coating, paint, plating, or galvanization to resist rust. Once that coating wears through, the base steel underneath corrodes, since carbon steel has little inherent corrosion resistance on its own. Stainless steel is different: adding chromium creates a thin oxide layer on the surface that resists corrosion without a separate coating, similar in principle to how aluminum protects itself. Aluminum also naturally forms a thin oxide layer on its surface that protects it from further corrosion, so it doesn't need a coating.²

Brass and other copper alloys are inherently corrosion-resistant as base materials, not because of a coating. Independent testing conducted by the Copper Development Association found that brass outperformed plated steel in accelerated-salt-exposure corrosion testing.³ This is a general material science finding, not a claim about any specific product's independent testing.

Unlike carbon steel, which develops rust when unprotected, brass may naturally darken or develop a patina over time. This surface change differs from corrosion, which weakens the underlying metal.⁷

Plastic

Plastic mail slot components are lightweight and inexpensive. Most standard plastics, without added UV stabilizers, degrade over time under sustained sun exposure, becoming discolored and brittle.⁴ That's a relevant consideration specifically for hardware mounted outdoors on a south- or west-facing door, which gets more direct sun over the years than an interior component would.

Where the Brass-Seal Fits

The Brass-Seal Forged Brass Mail Slot System is forged, not stamped, cast, or extruded, the other common ways brass hardware is manufactured. Forging compresses the grain structure of the brass, a manufacturing distinction from stamped sheet metal that's separate from any performance test.⁵ It's worth being precise about what that does and doesn't mean: the Brass-Seal's independently verified ASTM E283 results measure air leakage, a property of the sealing design, not a claim about the material's hardness or corrosion resistance. That distinction matters because the product uses brass, but only the sealing design has been independently tested.

Four materials, four different tradeoffs:

• Brass: naturally corrosion resistant as a base material, softer than steel, harder than aluminum
• Steel: harder than brass, though ordinary carbon steel needs an intact coating to resist rust, unlike stainless steel
• Aluminum: forms its own protective oxide layer, softer than steel
• Plastic: lightweight and inexpensive, prone to UV discoloration and brittleness outdoors

Frequently Asked Questions

Is brass more durable than steel for a mail slot?
It depends what's being measured. Steel is harder, but relies on an intact coating to resist corrosion. Brass is softer but corrosion resistant as a base material. Neither claim has been independently tested by BOTA specifically for the Brass-Seal. These are general material properties, not product-specific test results.

Does the Brass-Seal's material affect its ASTM E283 result?
The ASTM E283 test measures air leakage, not material hardness or corrosion resistance. The Brass-Seal's verified results reflect its sealing design, not a claim about brass as a material.

Is forged brass stronger than cast brass?
Generally, yes. Forging compresses and refines the grain structure of the brass, which reduces the internal voids and porosity that casting can leave behind, so forged brass typically shows greater strength and durability than cast brass of the same alloy.⁶ This is a general property of the manufacturing process, not a claim tied to any single product's independent testing.

Sources

1. Components For Industry. "Fastener Material Guide: Comparing Brass, Steel, Aluminum, and Titanium." blog.componentsforindustry.com
2. Fastmetals. "Metals with Corrosion Resistant Properties." blog.fastmetals.com/metals-with-corrosion-resistant-properties/
3. Copper Development Association. Corrosion testing of free-cutting brass vs. plated steel. copper.org/applications/rodbar/alloy360/corrosion_tests.php
4. Curbell Plastics. "Plastics for Outdoor Applications." curbellplastics.com/materials/operating-environments/plastics-for-outdoor-applications/
5. CT Forge. "Metal Stamping vs. Metal Forging: A Procurement Guide." creatorcomponents.com/news/metal-stamping-vs-metal-forging-a-procurement-guide.html
6. MetalTek. "Copper vs. Brass vs. Bronze: The Difference Between Alloys." metaltek.com/blog/copper-vs-brass-vs-bronze-the-difference-between-alloys/
7. MFG Shop. "Understanding Brass Oxidation: Causes, Effects, and Prevention." shop.machinemfg.com/understanding-brass-oxidation-causes-effects-and-prevention/

Related Guides

• Common Mail Slot Failure Points, and Why They Happen
• Mail Slot Buyer's Guide: How to Choose the Right Replacement
• What Is ASTM E283 Air Leakage Testing?
• Mail Slot and Building Hardware Glossary

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