Types of stainless steel and best finishing processes to get the best performance in coastal applications.
This article will outline the 2 most commonly used type of stainless steel (304 and 316) and their applications in coastal/marine environments where salt corrosion is a factor in the life and long term appearance.
Stainless steel is and iron based alloy that contains at least 10.5% chromium. Chromium is excellent at resisting corrosion. During the production of stainless steel chromium is added during the melting process along with other alloying elements such as nickel and molybdenum which work together to further enhance the corrosion resistance.
Composition of 304 and 316
- 304 stainless consists of 18% Chromium and 8% nickel
- 316 stainless has the same chromium and nickel with the addition of 2% molybdenum. This significantly increases the metal resistance to corrosion.
The finish or polished surface of the stainless also plays a critical role in how well it can resist corrosion. The smoother the surface the more resistant the stainless is to corrosion, for example, even though 304 is less resistant in its original manufactured state than 316, if polished to a mirror finished 304 may become more resistant than 316 which has an unpolished finish.
Surface roughness can be measured using an Ra reader, this instrument calculates the average surface roughens over a small sample area of about 10mm. The higher the readying the rougher the surface. A surface roughness of 0.4 to 0.6 Ra followed by electropolishing is a great finish when using 316 in coastal applications. To produce this finish the material must be mechanically polished.
Types of Finishes
Mill Finishes are the supply condition for all stainless steel flat products as they come from the mill:
- 1D – Hot rolled and annealed with mill scale removed. This is found on thicker sheet and plate, is slightly course and has low reflectivity. This is used for non-decorative purposes the appearance not very relevant
- 2D – is more refined than 1D and is achieved through cold rolling, heat treating and pickling. It has a matt surface and is used more for engineering purposes than visual
- 2B – This is similar to 2D bur has a final light roll using highly polished rolls to give a smooth surface. The smooth surface makes for excellent corrosion resistance when electropolished and is also the bases for machine polished finishes.
- 2R – This finish is uncommon however is the best mill finish as it is produced using highly polished rollers which make the material smooth and reflective.
Mechanically Polished Finishes can be achieved by polishing stainless flat and round products with different grit selections of sandpaper. The lower the number of the grit the courser the finish and vice versa, the higher the number the finer the grit. Each grit must be cycled over the surface in order to reach a higher grit. The higher the finish the more processes and hence the higher the cost.
- 240G – This is quite a course grit and the starting point for material to be sanded to when wanting to reach a higher grit finish
- 320G – This is not commonly used finish but must be processed to reach a higher git finish
- 400G – This should be the minimal finish for coastal applications. This produces a surface roughness reading of around 0.6 to 0.7 Ra
- 600G – this is a great finish for coastal applications it produces a surface roughness reading of around 0.3 to 0.4 RA. when following up with Electropolishing this finish produces the best finish without costs blowing out.
- 800G – If budgets allow both 800 and 1000G are exceptional finishes.
- 1000G – This is the highest finish available before mirror polishing.
- Mirror polishing – This is achieved using soft cloth material and polishing compounds and is the smoothest surface finish available prior to electropolishing.
Finding out the best type of stainless to use for a particular project and what type of finish is required should be discussed with an accredited fabricator who understands how stainless behaves when being polished to different finishes. The smoother the surface requirements and higher the finish means the cost of fabricating will be much higher. For example fabricating a mirror polished commercial balustrade hundreds of meters long and then electropolishing it could mean much higher costs than a client expects to pay. If requirements and longevity are properly understood a less costly alternative may be provided.
To get advise on your next stainless steel project with regards to design, material types and availability and finish please feel free to contact our expert team. If you already have a design we are also able to quote for manufacture and install for any project large of small.