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Anonymous Poster #1

Galvanic Protection Coatings

02/13/2016 12:09 AM

Can readers share thoughts on any new coating technologies to allow aluminium to be joined to stainless steel and galvanised steel without creating corrosion issues ? The old rule was " don't mix metals" but I have a job where I can save money for the elderly pensioner if I can find a coating to prevent these problems. As if it wasn't difficult enough , the location is near the ocean too. I need to make an aluminium wheelchair ramp and fasten it to the existing galvanised hinges , there are also 316L stainless fittings to bolt to the aluminium ramp. Any suggestions for the cheapest solution to use these materials together ? Is there any high tech spray on product that will kill corrosion issues for 5-10 years ? Can cathodic protection be 100% successful or would I need to get the hinges and fittings coated with something like dacromet ?

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#1

Re: Galvanic protection coatings

02/13/2016 3:13 AM

Try using inert washers and shims (néoprene, EPDM, nylon etc.) to prevent the dissimilar metals from touching. The only touching part will be the stainless steel bolts which will be ok as the size of the bolt with respect to the part means little or no corrosion to the hole in which it is fixed.

The galvanisation in a coastal location is not long lasting ; typically less than 5 years in the locations on the object where wetting / drying cycles occur frequently (such as a fixing). Don't use galvanised bolts as the galvanisation disappears rapidly leaving a rusty bolt.

You can put a sealant on the fixings but this has the effect sometimes of creating the anodic pool conditions for corrosion.

You can attach 316L to aluminium in a coastal location without too much corrosion to either ; perhaps a little superficial corrosion to the aluminium which is aesthetic only.

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#4
In reply to #1

Re: Galvanic protection coatings

02/13/2016 10:37 PM

You might consider a NEMA G-10 laminate and sleeves with stainless bolts.

Sleeves & Washers - Lamons

Isolating Washers (G10, G11, Phenolic, G3, Steel HC)...

Double Washer Set (per bolt/stud)

  • (2) each 1/8" thich steel ZP washer
  • (2) each 1/8" thick isolating washer
  • (1) each isolating sleeve
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#6
In reply to #1

Re: Galvanic protection coatings

02/14/2016 1:09 PM

I disagree with, "You can attach 316L to aluminium in a coastal location without too much corrosion".

While it is true that large areas of Al in contact with small areas of corrosion resistant (it is not stainless) steel may be tolerated in some environments, I say an oceanside location is not one, especially not with 316.

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Anonymous Poster #1
#7
In reply to #6

Re: Galvanic protection coatings

02/14/2016 3:23 PM

will Teflon coated bushes act as a barrier to any degree ? I can use those to seperate the galvanised parts from the aluminium and then nylon to isolate the stainless - aluminium joints. And how about bronze , can that be used as a barrier between galvanised steel and aluminium or are they stIll too close on that table ?

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#8
In reply to #7

Re: Galvanic protection coatings

02/14/2016 4:10 PM

You need three things to make a battery. Cathode, anode and electrolyte.

Galvanic corrosion will not occur in a dry environment, or between metals that are CLOSE on the galvanic series. Close is good.

Teflon would be my last choice because it is prone to cold flow under pressure, but might work in your particular case. Nylon would be better.

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#9
In reply to #7

Re: Galvanic protection coatings

02/14/2016 6:31 PM

Forget the nylon Just use duralac ( or tefgel ) Google it ��

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#11
In reply to #7

Re: Galvanic protection coatings

02/15/2016 11:42 AM

I live in a costal community and build sculptures that combine Stainless (it doesn't stain but it sure does rust) mostly 304 and 316 with 6061 aluminum. I often paint the aluminum. Both of these materials will corrode alone or together especially if you don't want them to. Galvanic corrosion will be greatly reduced by breaking the "circuit" in the battery that you are creating. I have used teflon for isolation and bearings for over 30 years on some things, and it is durable outdoors. Nylon, pfte, hdpe , neoprene,all work good to, although nylon swells when wet so you need to give it room. Paint films on aluminum are tricky, prep must be perfect, and the coating applied right after pretreatment or it will blister. Any scratch will also result in a blister or anodic pit. If the paint film isn't thick enough water will migrate through and cause blistering. Better to let it dull and oxidize in most cases. I sometimes use neutral cure rtf silicon on my connections that might be susceptible to water wicking into them. If you use stainless bolts get them black oxide coated, they will be more rust resistant as they won't gall off the chrome film whey assembling.

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#12
In reply to #11

Re: Galvanic protection coatings

02/15/2016 12:41 PM

"Stainless (it doesn't stain but it sure does rust)"

If it is rust, that is caused by iron contamination on the surface of the CRES.

That's why it is passivated or otherwise treated to remove all iron from the surface.

If CRES is stored on steel shelves, cut with steel blades or cleaned with a steel wire brush, it will rust.

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#13
In reply to #12

Re: Galvanic protection coatings

02/16/2016 2:02 AM

The rust that forms on 304 in coastal locations is not from iron contamination unless you are talking of localised contamination.

The IMOA has some good case studies : http://www.imoa.info/molybdenum-uses/molybdenum-grade-stainless-steels/architecture/stainless-steel-selection-system-case-studies.php

One shows the difference between 304 and 316 in coastal environment. I have also seen this myself in the south of France on a coastal path. It was clear that 304 had been specified due to the amount of powdery rust on the surface of the handrails.

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#15
In reply to #13

Re: Galvanic protection coatings

02/16/2016 9:20 AM

What we have, is failure to communicate. Or two people, separated by a common language. Maybe it's just me.

The "rust" that forms on 304 (and other CRES) in coastal locations is corrosion.

So we are arguing semantics.

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#16
In reply to #15

Re: Galvanic protection coatings

02/16/2016 4:11 PM

It was the "iron contamination" which made it seem as though the iron was coming from elsewhere than the actual stainless itself.

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#14
In reply to #6

Re: Galvanic protection coatings

02/16/2016 2:20 AM

There is an excellent set of tables for construction materials called BS 6484 - 1979 bimetallic corrosion. It has tables for each material and its reaction with every other material in a specified location.

Effectively, I hadn't seen that the aluminium table gives additional corrosion for aluminium in contact with austenitic stainless steel in a costal location (marine in the tables which means, to my mind, within 100m of the sea)

In mitigation, the fasteners in A4 stainless (316) are recommended for fixing aluminium and other dissimilar metals due to the small anode size of the screw.

A really good study on fasteners was done in 1976 :

david w taylor naval ship research and development center seawater corrosion fastener by harvey p hack

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#2

Re: Galvanic protection coatings

02/13/2016 4:51 AM

If you can design for a larger number of fasteners, non-metallic bolts, washers, and shims might work.

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#3

Re: Galvanic Protection Coatings

02/13/2016 8:21 PM

I've not had many issues with those materials, but it can happen!

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#5

Re: Galvanic Protection Coatings

02/14/2016 12:10 PM

Try the " PaintSquare" website----

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#10

Re: Galvanic Protection Coatings

02/15/2016 8:01 AM

A non-conductive adhesive would be a better bet.

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#17

Re: Galvanic Protection Coatings

02/19/2016 9:35 AM

My approach here on the oceanfront involves circuit potential blockage (insulation) and sealing against exposure to ambient corrosion atmosphere/liquids (isolation) with various nylon or polyolefin washers/sheets (insulation) and copious amounts of siliconized polyurethane caulking and epoxy coverage (isolation). It comes down to a design strategy for each of the typical bolted or screwed connections in your project. Of course, always use stainless fasteners, typically series 300 (best is 316) for bolts and series 400 (410 is usually the most available) for screws, albeit the later are more prone to developing a stable patina of rust on the surface.

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