Choosing the right screw in Portugal is often a decision between durability and cost. Although stainless steel A2 and A4 may look identical, the performance difference by the seaside is radical.
The Practical Choice: Where to Use Each?
INOX A2 (AISI 304): The Inland Choice
Stainless steel A2 is the workhorse of construction. It is excellent for humid areas, but it has a mortal enemy: salt.
- Where to use it: Interior of the country (Alentejo, Ribatejo, Trás-os-Montes) and urban areas far from the coast.
- Warning: It works well for garden furniture or small decks far from the ocean. However, if you use A2 within 5–10km of the Portuguese coast, you will likely see "tea staining"—ugly brown rust streaks — on your beautiful wood within a few seasons — especially in combination with some hardwoods containing high levels of natural tanning acids.
INOX A4 (AISI 316): O Especialista de Costa
A4 stainless steel is the undisputed champion for the Portuguese coastline. Known as "Marine Grade", it is specifically designed to resist the corrosive power of salt.
- Where to use it: Algarve, Costa Alentejana, Lisbon/Setúbal Region, Silver Coast - with the famous Ericeira, Beira Litoral and North Coast with a windy and rainy climate. Of course, Madeira and the Azores. And all the swimming pool areas where chlorine is used.
- Advantage: A4 Inox Contains Molybdenum, a chemical component that prevents corrosive corrosion caused by salt and chlorine. If the design is meant to last a lifetime by the Atlantic, A4 is the only option.
The Technical Deep Dive for Pros
When you're specifying for a large project or a high-end villa, the chemistry matters.
Composition and Resistance
The primary technical difference is that A4 contains 2–3% Molybdenum. This element creates a "passive" layer that is significantly more resistant to pitting—the tiny, deep holes that salt creates in metal which lead to snapping screws.
| Element | Inox A2 | Inox A4 |
|---|---|---|
| Common Designation | AISI 304 / 18-8 | AISI 316 / Grau Marítimo |
| Chromium (Cr) | 17% – 19% | 16% – 18.5% |
| Nickel(Ni) | 8% – 10% | 10% – 14% |
| Molybdenum (Mo) | 0% | 2.0% – 3.0% |
| Carbon (C) | ≤ 0.08% | ≤ 0.08% |
| Pitting Resistance (PREN) | ~18-20 (Moderate) | ~24-26 (High) |
| Salt Spray Resistance | Moderate | Excellent |
| Acid/Chemical Resistance | Low (avoid acids/chlorine) | High (resistant to chlorides/acid) |
Compatibility with Wood
Woods such as Oak, Cedar or Ipe have high level acidic tannins. The use of A4 stainless steel screws prevents the wood from developing black stains around the screw head, preserving the premium aesthetic of the deck.
Summary
- Within 5km of the sea: Always use A4 Inox.
- Swimming pools: Use A4 Inox (resistant to chlorine).
- Inland projects: A2 Inox is a safe, budget-friendly way to go.