Comparing AISI304 and AISI316 grades of stainless steel is important. While the stainless items in this website are mainly grade AISI 304 & AISI 316 – commonly referred to as G304 and G316 respectively. And while not rigorously enforced – “Marine Grade” is generally accurately applied to only G316.
Indoor vs Outdoor installation
G304 stainless steel is widely used and satisfy a broad demand for adequate performance at an affordable price. While, it has good corrosion resistance in a wide variety of environments, and is the best choice for INDOORuse.
G316 stainless steel has a higher level of corrosion resistance, and is often referred as “marine grade”. Typical applications are architectural components where they are exposed to extreme weather and climatic conditions, such as near the coast, industrial sites or swimming pools. It is usually the best choice for OUTDOOR use.
Difference in Alloys
While both grades are ‘silver and shiny’ – the devil is in the details. Here is a table of the elements required (along with steel) and the acceptable range of percentages that differentiate these 2 grades of Stainless Steel.
| Grade | C | Si | Mn | P | S | Ni | Cr | Mo |
| AISI 304 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.035 | ≤0.035 | 8.00 – 10.50 | 18.00 – 20.00 | — |
| AISI 316 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.035 | ≤0.035 | 10.00 – 14.00 | 16.00 – 18.00 | 2.00 – 3.00 |
Why is it ‘Stainless” Steel
Because is has a protective coating…. When stainless steel is produced, the chromium forms an outer oxide layer. As long as this layer remains intact, the stainless steel remains passive. And as long as the surface has unimpaired access to oxygen – it stays intact and self-heals. But deprived of oxygen the oxide layer begins to break down, the stainless steel becomes inactive and its corrosion resistance is reduced, and tea stains may be seen.
Stainless Steel “Tea Staining”
Firstly, stainless steel may suffer from a brown discoloration, or “tea staining” over time. Secondly, it is a visual impairment only, and does not affect the structural integrity or the longevity of the material. Secondly, the staining occurs most commonly within 5 kilometers from the sea and becomes progressively worse closer to the marine source. Finally, be aware that wind exposure, pollution and higher temperatures can create environments where tea staining might occur 20 kilometers or more from sea water.
4 Steps to “Keep it shiny”
- Specify the right Grade for the right environment
- Generally 316 Stainless Steel is much better.
- G316 is ideal for marine and highly corrosive environments.
- Choose a smooth surface
- Hand polished Mirror is better than Satin – Electropolished is best.
- The main cause of stain is salt deposited on the stainless steel surface. This is what deprives the stainless access to oxygen. To reduce the risk of tea staining, choosing or creating a smoother surface of the product is an ideal method.
- Install it Properly
- After installation, the completed structure should be washed and inspected from imperfection or contaminants caused in the installation process.
- If discovered, imperfections should be cleaned off and polished with a suitable stainless polish.
- Keep it Clean
- Stainless steel is not maintenance free but maintenance friendly – just give it a good rinse with the hose.
- When using stainless steel material outdoors, you need to clean periodically, especially in aggressive environments like coastal areas or swimming pools.
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