17

In the past 6-months or so, I purchased this set of stainless steel hex head bolt fasteners. In the kit, they require you to install the bolts using "anti-seize as a regular practice". The seller specifically states it is an "absolute requirement with aluminum!".

To this end, I'm wondering:

  • Why the absolute need to use anti-seize on these fasteners?
  • Are there metal to metal issues going on? (ie: Does SS gall aluminum?)
  • Are there any other issues when using SS fasteners over the stock fasteners?
  • I'm not looking for a why or why I should not be using these fasteners. That is all personal preference and in the realm of opinion. I would, however, be interested in any factual reasons for not using these types of fasteners, like they fail to deliver a torque load, or stretch over time, or what have you. Please back your answers with references. – Pᴀᴜʟsᴛᴇʀ2 Oct 29 '16 at 16:43
  • 1
    I would use anti-seize on any type of steel/aluminum bolted joint, for the same basic reasons as in the answers. The stock steel is likely to be a high-Chromium high-molybdenum steel (trade names like Chromoly) with less Chromium than SS but similar (though less severe) issues. – alephzero Oct 29 '16 at 21:40
  • This question got me wondering about the stainless steel fastener kits that you can get for aircraft. I've never purchased (or even seen) one, but I'd imagine that they would have similar issues. As best I can tell the kits give you replacements for some of the fasteners that get ugly over time (engine cowl, etc.) and they will almost certainly go into aluminum. Seems like the corrosion issues on these would be a big deal, so it is odd that the kits (that I've seen recently) don't mention this. – dlu Oct 30 '16 at 16:11
16

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:

  • 1
    Wow! Comprehensive and educational! Great answer! – PeteCon Oct 29 '16 at 18:17
7

The most immediate issue is galling. Stainless steel typically contains a lot of suspended carbides ie very hard particles in a soft matrix. This means that stainless threads are prone to galling and seizing.

This is further exacerbated with aluminium which is a relatively soft metal but with hard surface oxides also the difference in hardness between stainless and aluminium means that there is a big risk of deforming aluminium threads and although galling is a specific issue with stainless/stainless threads the relative softness of aluminium alloys means that lubrication is important even if you don't get actual galling per-se.

Even if the thread doesn't actually seize aluminium threads have a much lower load capacity than steel ones so lubrication will reduce the risk of tearing out the threads before the required torque is reached as the applied torque is a function of friction between the threads as well as the tension in the fastener.

Screw thread tend to have a very small area of contact between the mating surfaces and what can happen is that this creates local high pressure areas which strip off the surface oxides and defectively allow the two metal surfaces to weld to each other.

Copper based greases tend to be the most effective for alleviating this.

Galvanic corrosion is also a significant issue in this context but has already been well covered by another answer.

2

In addition to the other answers, if a specific torque is required during installation of these bolts, the friction coefficient between the threads is important. Bare steel on aluminum will likely have a high friction coefficient which means you may not have the correct bolt load. Check what the assembly procedure requires. Also of coarse make sure there is no dirt on the threads. If this is for a low load application, the friction coefficient wouldn't matter so much. You then just care to make sure the threads don't gall or corrode together so you can at some point in the future unthread without breaking them.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.