Is Stainless Steel or Aluminum with a Stainess Steel finish more durable?

[marcust]

I would like to know if Stainless steel 304 or aluminum with a stainless steel finish would be more corrosion resistant when outdoor 24/7/365 exposed to the sun, rain, snow, etc. but not salt. Not a coastal climate. Both materials are resistant to corrosion and are sold as fixtures, etc. but I have not been able to find out which would last longer in this situation

 

[PAllen]

Purely based on general logic (no expertise in this), I would think the stainless steel. Simply because if a finish became compromised, however it might happen, the underlying aluminum would the be quite vulnerable. In contrast, there would be no significant failure if the stainless steel developed a pit.

 

[marcust]

I've seen gutters last a VERY long time, with the finish never coming off after over 40 years, if the finish is similar to how gutters are done, what would you think?

But this question is not about gutters though it's just something I've noticed.

 

[essenmein]

What do you mean by "aluminium with stainless finish"? Polished? Some sort of plating? Paint?

I think in general the right stainless alloy (304 or 316) would easily outlast anything made from Aluminium.

 

[anorlunda]

Aluminum masts on sailboats last indefinitely. They are anodized, or painted, or powder coated.

For some applications, galvanized steel is better than stainless steel.

So, what is your application? What is the context of the question?

 

[marcust]

Generally curious about exterior lights, both would essentially be same stainless steel color, would expect some fading but not looking for ANY corrosion. It is a commercial application so needs to last 20+ years easy hopefully over 50 if I maybe add a coating to it.

I've seen steel lights with stainless finishes deteriorate in a year or so so I don't want that issue.

All I know about the aluminum fixture is that it has a stainless steel finish, don't know if it is anodized, powder coated, but I would guess painted since it's the cheapest option I would assume.

I know the stainless fixture is 304 stainless I've thought about maybe getting citrisurf 77 while new and passivating it before the install and then add a clear coat of rustoleum clear enamel. Any suggestions to extend life on either would be great.

 

[anorlunda]

Do you mean the reflector for the lights that must be shiny? If not, then you should also consider non-metallic materials.

Corrosion is not the only consideration, UV exposure also limits the lifetime of outdoor devices.

 

[essenmein]

As far as architectural materials, here is some more info:

https://blog.dahlstromrollform.com/best-metals-for-exterior-applications

 

[marcust]

I am not worried about the shine I just want it to retain its semblance of stainless steel color and not corode, fading over time is not an issue. Unfortunately these are the best materials I have found everything else is just steel (painted or powder coated) so these two options seem to be the best.

That is a good article.

I am leaning towards the stainless steel. Any suggestions on passivating it or coating it with a clear enamel? This or any type of maintenance will only be done once before installation, there will be no yearly maintenance after installation, ever.

 

[essenmein]

Both Aluminium and stainless rely on an oxide layer to prevent further corrosion, covering the metal will initially prevent the metal being attacked, but also has the disadvantage of preventing the oxide layer from maintaining itself.

This means that when the coating inevitably fails, it can corrode quite aggressively, esp if water for example is able to get in between the coating an metal and create an oxygen starved environment, now the oxides can't build and corrosion happens at an accelerated rate (more generally known as crevice corrosion: https://en.wikipedia.org/wiki/Crevice_corrosion ).

Eg:

 

[marcust]

Thank you, I had thought of something similar to that as I've seen it happen before to steel. I will not coat it then since coat lasts maybe a year max.

Also just found out the aluminum fixture is made with a mix of aluminum and stainless steel, not sure how but they use both metals. I am guessing pure stainless steel would be better here correct?

 

[essenmein]

If the two metals are not electrically isolated from each other, stainless+aluminium will have major issues with galvanic corrosion.

 

[delacruzroberta]

Aluminum is typically not as strong as steel, but it is also almost one third of the weight. This is the main reason why aircraft are made from Aluminum.

Stainless steel is made up of iron, chromium, nickel, manganese and copper. The chromium is added as an agent to provide corrosion resistance. Also, because it is non-porous the resistance to corrosion is increased. Aluminum has a high oxidation and corrosion resistance mainly due to its passivation layer. When aluminum is oxidized, its surface will turn white and will sometimes pit. In some extreme acidic or base environments, Aluminum may corrode rapidly with catastrophic results.

Aluminum has a much better thermal conductivity (conductor of heat) than stainless steel. One of the main reasons it is used for car radiators and air conditioning units.

Aluminum is typically cheaper than stainless steel.

Stainless steel is stronger than Aluminum (provided weight is not a consideration).

Stainless steel is less reactive with foods. Aluminum can react to foods which may affect color and flavor.

 

[marcust]

Both Aluminium and stainless rely on an oxide layer to prevent further corrosion, covering the metal will initially prevent the metal being attacked, but also has the disadvantage of preventing the oxide layer from maintaining itself.

This means that when the coating inevitably fails, it can corrode quite aggressively, esp if water for example is able to get in between the coating an metal and create an oxygen starved environment, now the oxides can't build and corrosion happens at an accelerated rate (more generally known as crevice corrosion: https://en.wikipedia.org/wiki/Crevice_corrosion ).

Eg:
View attachment 252789

This was very helpful to me. I decided to not use a coating. I have another question for you and the rest of the people here. With galvanized steel that is for outdoor use, that is also POWDER COATED at a high temp and bonded properly, will it have this same effect (crevice corrosion) or will it not because the powder coat material is meant to stop corrosion?

I don't know if powder coating actually has a similar effect like stainless steel, aluminum, or zinc as far as the oxidization process or not but would like to learn more about it.

 

[marcust]

Aluminum is typically not as strong as steel, but it is also almost one third of the weight. This is the main reason why aircraft are made from Aluminum.

Stainless steel is made up of iron, chromium, nickel, manganese and copper. The chromium is added as an agent to provide corrosion resistance. Also, because it is non-porous the resistance to corrosion is increased. Aluminum has a high oxidation and corrosion resistance mainly due to its passivation layer. When aluminum is oxidized, its surface will turn white and will sometimes pit. In some extreme acidic or base environments, Aluminum may corrode rapidly with catastrophic results.

Aluminum has a much better thermal conductivity (conductor of heat) than stainless steel. One of the main reasons it is used for car radiators and air conditioning units.

Aluminum is typically cheaper than stainless steel.

Stainless steel is stronger than Aluminum (provided weight is not a consideration).

Stainless steel is less reactive with foods. Aluminum can react to foods which may affect color and flavor.

Thank you! Honored to be the reason of your first post here lol

 

[Joe591]

If cost is not an issue I think you should go for the 304 stainless. I worked for a company where we made aluminium radiators and stainless tanks. Aluminium does corrode and it can become quite visible. Sometimes when people want corrosion resistant options that are cheaper than stainless they go for Aluminium, like toolboxes for boats, but stainless is superior in the end. 304 stainless is very good. The only time you need 316 is for marine applications (salt water), food applications like dairy and strangely enough CO2 piping. Apparently 304 stainless and CO2 don't like each other.

 

[hutchphd]

With galvanized steel that is for outdoor use, that is also POWDER COATED at a high temp and bonded properly, will it have this same effect (crevice corrosion) or will it not because the powder coat material is meant to stop corrosion?

I have had very good outdoor longevity with galvanized steel painted with alkyd enamel. No crevice corrosion.

 

[Tom.G]

It appears that both Aluminium and Steel are acceptable. Here is a link to the 2009 handbook that the city of Los Angeles, CA, USA uses as a purchase specification for street lighting.
http://bsl.lacity.org/downloads/business/bluebook2009r3.pdf

Cheers,
Tom