Stainless Steel 316 at Marina Sea Coast

[IN75]

I have a project on an island seashore. It is in a very harsh environment where it is very hot, high humidity and corrosive.
I use SST 316 machined parts as fixing brackets for anodized Aluminum Pipes (in direct contact).
I observed some Brown Rust Spots at the SST surface.
I want to know the following:
1- Can we consider this as a serious corrosion case.
2- What is the machining effect on SST 316? Do we need to have a polished surface if the rough machined surface may catch more corrosion.
3- Do we need to use some insulation between Aluminum and SST to avoid any galvanic corrosion, keeping in mind that size of SST bracket is very small compared to Aluminum.
4- Will the Acid Pickling and Passivationon be a helpful treatment for future jobs or do we need another coating?
5- Can we consider a guarantee (corrosion free product) of 15 years for SST 316 for such application.

 

[YLWEA]

In atmospheric service there is not likely to be significant galvanic corrosion except right at the interface between the two materials, and only when there is liquid water (dew, rain, etc.) in contact with the metal at that point. The relative area of the two metals doesn't make a lot of difference in atmospheric exposure, but it is a big deal if immersed below a water line.

It is not uncommon for rust spotting on Type 316, particularly if the surface was ground or wire brushed with wheels previously used on steel or iron. Small particles of iron can get transferred to the stainless surface and corrode and initiate pitting on the stainless. Foreign material like dirt or salt deposits can also initiate staining. Keeping the surface clean, using a smooth finish (not a rough ground one)and passivating the stainless are all likely to reduce the occurrence of staining.

The atmospheric corrosion is usually cosmetic and not structurally significant. You might get a warranty against structural failure, but probably not against superficial staining and/or pitting because there are too many things that are too difficult to control in the service environment.

 

[mcguire]

316 does not have enough chromium and molybdenum to resist minor surface corrosion in a warm chloride environment unless the surface is in optimal condition. The following controllable variables can minimize superficial corrosion.
1. Keep sulfur content below 0.003%. This means no free machining versions of 316.
2. Pasivate the surface. There are not-too-toxic passivating solutions sold, usually citric acid based.
3. Minimize residual stress from fabrication.

If you want to guarantee for 15 years, you are at risk with 316. You may need to upgrade to 2205. The 316 may be saved by the aluminum acting as a sacrificial anode, but I would need to study the overall design before I would sign off on a life span.

 

[EdStainless]

2205 is a good option. It has better corrosion resistance and higher strength.
If you don't mind a dull finish a pickling treatment would be the best.
I have seen many posts break off at their mounting from this kind of effect.

Corrosion never sleeps, but it can be slowed down.