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Old 03-31-2006, 08:21 PM   #1
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Question Chemical reaction?

Both of us having long forgotten all science/chemistry that we ever knew, have a question........... will stainless steel chemically react over time with aluminum?

Inquiring minds want to know
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Old 03-31-2006, 08:28 PM   #2
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Quote:
Originally Posted by silver 67
Both of us having long forgotten all science/chemistry that we ever knew, have a question........... will stainless steel chemically react over time with aluminum?

Inquiring minds want to know
Silver 67,

Are you speaking of geological time or human life span? As my geology teacher said "We have all the time in the world"

Bill
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Old 03-31-2006, 08:36 PM   #3
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Cool What is time?

Time and space - the age old question...

I think we are concerned about the life of either us, or the Airstream, and since it's vintage and we are too, I would guess about 20 years....
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Old 03-31-2006, 08:42 PM   #4
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If in a dry environment, with large aluminum surface compared to SS, you should be ok. Example: SS screws in aluminum sheet would be OK.

In a wet environment, especially marine, and with the opposite surface area effect (aluminum rivets in SS sheet) the results would be catastrophic.

Looking at the galvanic series, active SS is closer to aluminum than passivated SS. So passivation would not be desirable.
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Old 03-31-2006, 09:02 PM   #5
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hi silver67

sure they react but why not desribe the application/issue......

if it's just marker light screws....i wouldn't worry....and the screw threads can be coated with spoke/nipple wax....or other things....not locktite.

if you are thinking about stainless sheet metal...like interior skin....it would be easy to insulate the contact...

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Old 03-31-2006, 09:07 PM   #6
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Specifics...

Application - 1/4 stainless steel bolts in exterior door hinge....
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Old 03-31-2006, 09:09 PM   #7
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Corosion

Beth-
Al and stainless steal will react over time. In the presents of salt water (or any electrolyte) a galvanic cell will form with Al acting as the anode and suffer from accelerated corrosion. The corrosion rate is strongly influenced by surface area ratios and the amount of oxygen diffusion in the electrolyte as well as the difference in potential between the metals. I don’t remember all the details, but I think this is best shown with the Tafel equation…I’ll post an update after I find my notes…and find the values…
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Old 03-31-2006, 09:11 PM   #8
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One fact that can confuse this matter is that just about every modern production sailboat has an aluminum mast and boom, with stainless steel fittings (tangs for shrouds and stays, gooseneck, crosstree fittings,etc), attached with stainless steel rivets and screws, so why don't they fall apart in the salt spray??
Well, for one thing, the aluminum extrusions are anodized, which coats the outside of the extrusions with a very hard layer of aluminium oxide, and this acts to prevent further corrosion. In addition, between the stainless steel and aluminum, it is good practice to place an electrical insulator such as zinc chromate paste or a gasket, and to dip the fastenings in the paste. The 32 foot aluminum mast on my sea-going yacht shows no visible corrosion, 30 years after I built it.
If we filled a deep swimming pool with the right electrolyte we could anodize a complete Airstream body shell......
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Old 03-31-2006, 09:20 PM   #9
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Quote:
Originally Posted by JoshuaTonbo
Beth-
Al and stainless steal will react over time. In the presents of salt water (or any electrolyte) a galvanic cell will form with Al acting as the anode and suffer from accelerated corrosion. The corrosion rate is strongly influenced by surface area ratios and the amount of oxygen diffusion in the electrolyte as well as the difference in potential between the metals. I don’t remember all the details, but I think this is best shown with the Tafel equation…I’ll post an update after I find my notes…and find the values…
All I want to know is should I use stainless steel bolts in an alum. hinge. If not maybe plated bolts?

David
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Old 03-31-2006, 09:23 PM   #10
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Use stainless steel bolts, with a rubber gasket between the skin and the hinge, and coat the shank of the bolts with zinc chromate paste, or dielectric grease, or similar. Fibre washers would be good.You'll be fine.
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Old 03-31-2006, 11:24 PM   #11
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Quote:
Originally Posted by silver 67
All I want to know is should I use stainless steel bolts in an alum. hinge. If not maybe plated bolts? David
the plea of a craftsman who didn't want to build the watch, he just wanted to know what time it was... OK, OK, I'm an engineer, too, but I never miss the humor (if I'm not the one doing the dispensing....).
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Old 04-01-2006, 08:49 AM   #12
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Quote:
Originally Posted by silver 67
Both of us having long forgotten all science/chemistry that we ever knew, have a question........... will stainless steel chemically react over time with aluminum?

Inquiring minds want to know
with much respect to the sailboat crowd, the answer is NO, not in your lifetime.

at the power company we deal with 3 types of metal for line hardware and conductors.

copper, aluminum and stainless.

i have removed stainless bolts that have been in direct contact with aluminum for over 50 years. no corrosion, no problems.

i think they will hold your door on for how ever long you want with no problems.

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Old 04-01-2006, 09:11 AM   #13
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John, my supporting reference for this is Wikepedia :
Galvanic corrosion

Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of the less noble material. This effect can be prevented by electrical insulation of the materials, eg. by using rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or keeping the size of the less-noble material significantly larger than the more noble ones (eg. stainless-steel bolts in an aluminum block won't cause corrosion, but aluminum rivets on stainless steel sheet would rapidly corrode.
[edit]

The whole encyclopedia entry for stainless steel is at:
http://en.wikipedia.org/wiki/Stainless_steel

This accords with my own experience and understanding of this complex subject, particularly with respect to the different grades of stainless steel, the presence of oxygen, and the relative sizes of the dissimilar metals, as Don said earlier.
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Old 04-01-2006, 09:19 AM   #14
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nick

i don't disagree at all!

i would venture to guess the hardware i use at work is very high grade and non magnetic. therefore the lack of problems.

for trailer use i have replaced all of the zinc plated screws on vents, clips and lights that were rusted with stainless. with no ill effect so far.

the stainless fasteners i buy from ace hardware are non magnetic as well. i know because my handy dandy pick up tool won't grab them when they roll under my workbench!

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Old 04-01-2006, 09:22 AM   #15
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Quote:
Originally Posted by john hd
with much respect to the sailboat crowd, the answer is NO, not in your lifetime.
at the power company we deal with 3 types of metal for line hardware and conductors.
copper, aluminum and stainless.
i have removed stainless bolts that have been in direct contact with aluminum for over 50 years. no corrosion, no problems.
i think they will hold your door on for how ever long you want with no problems.
john

I'm with John on this. I have not experienced any problems in 25 years of work on power systems with aluminum and stainless connections. With current flowing at various rates galvanic erosion is a big problem if the connection is made with the wrong materials.
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Old 04-01-2006, 09:30 AM   #16
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for trailer use i have replaced all of the zinc plated screws on vents, clips and lights that were rusted with stainless. with no ill effect so far.
J, that's just what I've done. I just fail to understand why Aistream doesn't do this on assembly. The cost difference in bulk buying would be small, and it would be a marketing point to differentiate a "premium" product from SOBs. Some forum members report corrosion from zinc-plated steel screws on virtually new units. In my UK Screwfix catalogue, some sizes of stainless screws were cheaper than plated!
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Old 04-01-2006, 10:19 AM   #17
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The process you are worried about is called "dissimilar metals corrosion"! The Military worries about this a great deal on aircraft and they handle it by coating between the metals with "zinc chromate". As used by the military it is a yellow color, but I think you can find it in other colors.

Yes, it can become an issue over very few years. It has nothing to do with the Galvanic Process. No current need be present to cause it.

We once rebuilt an aircraft and prior to painting it OD, we painted it with zinc chromate undercoat and flew it around a while. Looked wierd.
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Old 04-01-2006, 10:49 AM   #18
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[quote=cooperhawk]The process you are worried about is called "dissimilar metals corrosion"! The Military worries about this a great deal on aircraft.Yes, it can become an issue over very few years. It has nothing to do with the Galvanic Process.quote]

My humble understanding is that these are two names for the same phenomenon. With respect to airframe corrosion, this is described at http://www.avweb.com/news/columns/189857-1.html

"Another kind of airframe corrosion occurs where dissimilar metals come into contact -- for example, where steel screws are used to fasten aluminum parts, or where a stainless steel firewall is riveted to aluminum structural members. This is known as "galvanic corrosion" or "dissimilar-metals corrosion." "

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Old 04-01-2006, 10:59 AM   #19
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Quote:
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nick

the stainless fasteners i buy from ace hardware are non magnetic as well. i know because my handy dandy pick up tool won't grab them when they roll under my workbench!

john
Sorry, but Stainless Steel is never magnetic - all types and grades. The magnetic properties of a metal have nothing to do with the di-electric corrosion between alumnium and stainless steel. It's the electric current produced between two different metals that are in direct contact with each other.
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Old 04-01-2006, 11:09 AM   #20
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Sorry, but Stainless Steel is never magnetic - all types and grades.
The martensitic grades are magnetic.

The magnetic properties are a result of the amount of nickel in the alloy, and therefore are related to the corrsion properties.
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