galvanic corrosion

Is using 300 series stainless steel hardware to bolt to hot-dip galvanized steel a bad idea in an out door environment? I'm finding
conflicting information on the cathode/anode potential of various materials and how close they need to be. Also this will be in Oklahoma, no where near salt water.
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Chris W
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Dear Chris W:

You will have galvanic corrosion. You can only control the rate of corrosion.

It still rains in Oklahoma. Different grades of stainless steel bolted together will even corrode. The important thing is to "break the circuit". Place a non-conductor between the metals, even if you don't insulate the bolt(s). Design the points of contact to drain thoroughly.
David A. Smith
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Galvanizing (zinc) is sacrificial to steel. Rather than stainless steel bolts, you could consider galvanized nuts and bolts which are easily available. Hot dip galvanizing is an expensive but very satisfactory way of preventing steel from rusting for many years - given the coat is not rubbed or abraided. Electro plate zinc can be much thinner, and accordingly less durable. Generalizing on the Oklahoma environment - you will notice that cars do not rust but rather wear out. That's a relatively benign environment with low humidity, and infrequent rain.
Brian Whatcott Altus OK
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Chris W wrote:

I used to work with the helium pipeline which runs from Otis, KS to Amarillo, TX (all the way across OK). The buried pipeline has regularly spaced pieces of metal bolted to it to inhibit the corrosion of the pipeline. These pieces of metal are regularly inspected and replaced.
The idea is to *use* galvanic action. The more active metal will react with water/oxygen first. Zinc is more active than iron, so the 'galvanizing' process (dipping steel in molten zinc) was developed to prevent the corrosion of mild steel. Stainless steel is essentially just as active as mild steel. It resists corrosion on its own through a different process: the secondary metal (usually nickel or chomium or some other transition metal) with which the iron is alloyed forms an oxide which coats the metal and does not come off as easily as iron oxide. When iron oxide (rust) comes off it exposes fresh metal to corrosion.
Corrosion can occur wherever metal, water, and oxidizers are present together. Oklahoma is not dry enough, nor is the bottom of the ocean isolated enough from oxidizers. Corrosion is significantly *slowed* by temperatures that never exceed 0 C, and may be kinetically hindered when the metal is packed in ice, but iron can rust almost anywhere. Of all the metals found on the earth, only a few are found naturally in an unreacted state - gold, silver, copper, tin, lead. The rest, including iron, have reacted with their environments and are no longer found in quantity as unreacted metals - i.e. they are already fully 'corroded.'
Google "cathodic protection (535,000 hits).
Tom Davidson Richmond, VA
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Stainless steel corrodes when brought into contact with iron. It's okay for the iron to oxidize, but why and how does that affect the stainless material?
w.
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Dear Helmut Wabnig:
wrote in message

As another has posted, the protective effects of stainless steel derive from an oxide layer of chorium or nickel or both. If you contaminate a surface with free iron, corrosion will proceed from there despite the nickel and chromium. You can see where steel tools tightened bolts, scuffed against stainless steel surfaces, regardless of the salt content in the air. Passivation removes the surface iron, leaving enough chromium/nickel to act as the protective layer. Strangely enough buffing a surface with a non-ferrous abrasive has a similar passivating effect.
David A. Smith
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How long are the stainless steel bolts in contact with the steel that you are galvanizing? I ask this because unless the stainless is in nearly constant contact vs 5 minutes of each hour, you may be fine using stainless for a long time (assuming that things are cleaned up after each part). Then, what is the life expectancy of the equipment? Also, what are the economics of having some "throw away" attachment hardware vs corrosion inspection/control.

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YouGoFirst wrote:

I guess I wasn't clear in my first post. I have a galvanized steel antenna tower that I got used. I don't have any hardware to bolt it together with. I was wondering if stainless steel hardware would be a good choice. I need some 1/4 5/16 and 3/8 nuts and bolts and you can't get all of them in hot dip galvanized but I can get them all in stainless steel. I found some grade 8 hardware that has what they are calling "Ultra coat" that is supposed to be better than hot dip galvanized so I may just use that.
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Chris W
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I wouldn't worry about it. Galvanic damage occurs with the ratio of the areas exposed. A cathodic bolt in an anodic material will not corrode nearly as much as the opposite, because the area of the bolt is so much less than the area of the structure. A general plus...if you ever have to take it apart, the SS will be much easier to disassemble.
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Harry Andreas
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Have you tried the spray galvaniz paint ofer the top of everything after it is assembled? We used that at the satellite station where I worked. It's another sacrificial element, but not structural, so when it goes away nothing bad happens.
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