Stainless and aluminum are pretty far apart in the galvanic series of metals with aluminum at the anodic (corroded) end. This means that in the presence of an electrolyte the stainless bolt will be acting as a cathode pulling electrons from the anode (the aluminum threads) and causing them to corrode.
Since there isn't much metal in the threads, and since a thread failure would be difficult and/or expensive to repair an isolation compound can/should be used when assembling. In the marine world Lan-o-cote (anhydrous lanolin) is often used. Some folks use Duralac or Tef-gel which serve a similar purpose with the additional benefit of that warm fuzzy feeling that comes from paying a premium price – in other words, I know of people who swear by them, but not of any independent data that backs them up…
Stainless steel is also vulnerable to crevice corrosion which can happen when the fastener is cut off from oxygen (say threads immersed in oil). Stainless needs oxygen to maintain its passive surface layer. Protection against crevice corrosion usually involves either selecting a more corrosion resistant grade of stainless or using sealants to keep out the corrosive agent. I would think that you could pretty much count on the surface layers of stainless fasteners getting damaged as the fastener is torqued.
According to Wikipedia aluminum galls easily so that might be another reason for the emphasis on using an anti-seize compound.
The other issue would be comparing the strength of the stainless replacements to the original fasteners. I've read claims that stainless is, in general, not as strong as an equivalent steel bolt. But I'm not knowledgable enough about metallurgy to understand what I find on this beyond "beware, stainless is not always an upgrade."
References and Articles:
While the two most popular grades of stainless steel might seem identical at a glance, they hold important differences that can lead to substantial savings over the life of your product depending on your intended uses.
All stainless steels contain a mix of iron and chromium at various levels. But the exact mix—and other additions — are what create the differences between grades.
The most popular 304 grade stainless contains 18% chromium and 8% nickel — though other alloys exist within the same grade.
Stainless steels of the 316 grade include less chromium — often around 16% — but increase nickel levels and add molybdenum to the mix.
In most common alloys, you’ll find 10% nickel and 2 to 3% molybdenum. However, specialty alloys might have as much as 9% molybdenum.
Most stainless steel ordered around the world is Grade 304.
It offers the standard corrosion resistance, formability, strength, and easy maintenance for which stainless is known.
While 316 comes in second in terms of quantities sold, it offers vastly superior corrosion resistance to chlorides and acids.
This makes it popular for a wide range of environments—including:
These increased resistances also help to take the guesswork out of cleaning and allow you to clean your stainless more often using stronger cleansers and detergents with little worry of damage or altering the appearance of the stainless steel.
This makes 316 an ideal upgrade for environments with strict hygiene and cleanliness standards.
With the benefits offered by 316 stainless steel, you might wonder why 304 remains the most popular…
If you’re not sure which to choose, consider these common uses for the two grades:
In short, it’s mostly about cost and corrosion resistance requirements.
If you don’t need the added resistance of 316, standard 304 grades will offer similar performance and longevity at a lower price point.
However, in situations where the added resistance is needed, 316 often improves the functional lifespan of the steel products by years.
This can lead to drastic long-term savings despite the initial increase in investment.
Looking for more information on the various traits and performance considerations surrounding other stainless steel grades?
By matching the grade of stainless to your intended usage and environment, you can optimize both the financial and physical benefits of your stainless steel investment.
If you’re not sure where to start or are looking for a trusted partner with a wide selection of stainless options, the experts at Unified Alloys have more than 40 years of experience supplying industries across North America. Call today for a consultation!