Aluminum Fasteners

[honkonbobo]

A friend and member here (TT) replaced his with stainless several years ago, zero issues and screws look like new...

I have been replacing the screws in my camper with stainless when needed or adding mods,used sealant to prevent water intrusion. After 5 years there is no visible corrosion...

as above, a lot of people report success with stainless screws based on the absence of corrosion on the screw head.

my understanding is any corrosion would be at the point where the metals meet (assuming conditions were right for the corrosion to occur) which means its possible for the stainless heads to appear completely corrosion free but the aluminum around the screw hole to be in very bad shape? not saying their screws aren't doing the job... just that they are not measuring success in the right way.

i appreciate the comments and have enjoyed the discussion. i haven't ordered the new screws yet but will very soon now as the camper is going into the garage for the winter shortly and i am leaning toward either being the guinea pig for the aluminum screws or going with zinc plated screws with EPDM washers (thanks for that Craig).

posters have commented that "aluminum isnt as strong as steel." i think we all agree on that but the question here isn't if aluminum is as strong as steel but rather if its strong enough and as i said i am not planning on using them for anything structural like jack brackets/strut mounts/etc. i don't need to kill a mouse with a shotgun if i mousetrap will do fine.

the strategy with the zinc screws is really to let the plating on the screws corrode over time (probably very very slowly with zinc plating, EPDM washers and sealant coated threads) and replace them down the road again in the name of preserving the aluminum frame.

wow am i really getting this into camper screws?

 

[ckent323]

At risk of being pedantic and pedagogical:

Galvanic corrosion (also called bimetallic corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.

Note: an electrolyte must be present. Water is an electrolyte. No electrolyte no corrosion. If the path for water to get to the stainless steel screw thread interface with aluminum frame is blocked by a coating (e.g. Zinc) or sealant, even a thin film of sealant, then there is no galvanic corrosion.


" Although aluminium is anodic to stainless steel, large relative surface areas of aluminium to stainless steel can be acceptable, dependent on local conditions.

Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.

An example of the safe use of stainless steel and aluminium together is where stainless steel fasteners and hold down bolts are used to secure aluminium roadway or bridge parapet guards.
Even with no insulation between the metals, there should be little risk of corrosion.
In contrast, in a marine environment, severe localized pitting corrosion to the aluminium treads has been observed where un-insulated stainless steel bolts were used to secure the treads in place.
On the same ladder however, bolts with sound insulating washers did not show any pitting on the surrounding aluminium.
This illustrates the beneficial effect of breaking the corrosion cell by isolating the two 'dissimilar' metals in marginal cases."

Ref: https://www.bssa.org.uk/topics.php?article=89


I hope this is helpful.

Craig

 

[longhorn1]

Steel fastener with zinc coating

 

[ntsqd]

Or a silicon-bronze alloy fastener for lower strength requirement joints, but I'm in the zinc plated steel camp too. As a bonus you generally get a higher strength fastener when buying plated steel over stainless. Just buy SAE Grade 5 or better and it will be stronger than SS. I just wish that good plating was still done on regular fasteners.

 

[a8ksh4]

Looking through fasteners for sheet metal on mcmaster, these are the options I see:

• Zinc Plated and Black Zinc-Plated—Provide corrosion resistance in wet environments.
• Black Ultra-Corrosion-Resistant Coated—More corrosion resistant than zinc plating and provides a decorative finish that won't rub off.
• Ultra-Corrosion-Resistant Coated—Resists corrosion better than zinc plating and minimizes reactions between dissimilar metals.

Any idea if the ultra-corrosion-resistant coated is actually better than zinc (galvanized) in our case? I'd only be guessing at what the coating actually is.