Galvanic Action between Galvanized and Stainless Steel?

A. I wouldn't do it, especially in very corrosive environments, because as you say, the zinc is sacrificial, and it will dissolve. After it is gone, there is nothing stopping the remaining steel from corroding with the stainless as the cathode.

A. I think the problem you fear, and which Tom confirms, would be real: spot corrosion of the beam in the area near the clamp. However, your closing sentence about self-sacrificial zinc probably isn't accurate. The fact that the corrosion occurs near the contact point rather than across the whole beam is probably a practical issue rather than a theoretical one. In theory, the corrosion would not be localized to the area of contact; rather the anodic current would be spread across the entire surface of the beam. But in the real world, a bit of water probably collects at the beam clamp rather than immersing the entire beam, so the galvanic current is restricted to the wetted area.

A. Jeremy,

As Ted and Tom imply, you could well have problems especially if the contacts got wet!

If you are concerned, why don't you play safe and, if you can, place some plastic all around the contact area? In other words chemically isolate that area. Another option is to 'paint' the contact points of your clamps with a suitable paint.

You will definitely have some corrosion or preferential corrosion if you join dissimilar metals together, that's the nature of the beast!

I am slightly biased towards plastics (40 odd years in this field) but the plastic clamps have nowhere the load bearing capacity of metal ones.

Food fer thought, as they say in bad English ... but you have an excellent command of this universal 'linqua franca'.

Cheers !

A. Zinc in contact with stainless steel is extremely dangerous in time of fire. There is a chemical reaction and that can cause an explosion. That is why stainless steel thermowells having union-nipple-union construction should NEVER be specified with galvanised union-nipple-union fittings, only black iron or stainless.

A. Zinc will cause cracking of SS in case of fire (high temp.)

A. If you couple zinc with stainless steel near the sea, you can get embrittlement of the stainless steel. This is due to the formation of atomic hydrogen when seawater breaks down and releases chlorine molecules.
As zinc is amphoteric it can be attacked by either very low or very high pH solutions. The hydrogen gas that forms can enter the molecular matrix of the stainless steel and expand, causing the steel to crack.
But please note that this normally only happens when the steel is under stress.
I have coated stainless steel with a zinc silicate many times over the years, and once the silicate was coated with a polyurethane primer and paint there was never any problem as the paint was a 'salt barrier' to the metals.

A. The materials are actually very far apart galvanically. Zinc (the galvanized finish) is very cathodic to stainless steel and unless the area of the stainless steel is fairly small, it will accelerate the corrosion of the galvanized finish.

A. Galvanised steel uses zinc as a sacrificial anode to protect the base metal. Stainless steel will be cathodic to the steel, so it will be even more cathodic to the zinc, hence you will get enhanced corrosion of the zinc. I would suggest you put plastic washers in the break any corrosion circuit

Thank you for the responses.

A. My interpretation of the advice from the British Stainless Steel Assn. (see their site) is that this is likely to cause problems unless isolating washers are used. Fixing galvanized brackets using stainless nuts/bolts would probably be OK.

A. I would recommend that you do put an insulating element between them even indoors. You cannot always control the humidity. What will the RH be in the building? If you can be absolutely sure that there is no moisture in the contact area you might get by with it. What kind of fasteners will be used to connect the two? That could cause you more problems.

A. Insulating is to protect your galvanized zinc coating, not to protect the s.s. If your stainless steel surface area, compared to galvanized is small, and if you coated either of them, I believe it would be o.k.

A. In addition to concerns regarding galvanic transfer, liquid metal embrittlement must be considered in the event of a fire. Zinc attacks austenic material starting at 420 °C. The severity of the attack and consequences depend on heat and the stress level within the member. At 570 °C on highly stressed members the attack may be severe enough to result in rupture. I am not an expert, but please consult one. I advise this not because of the likelihood of an event, but because the failure mode could result in a sudden collapse as opposed to a progressive collapse that allow a safe exit for occupants of the structure.

A. There can be different metal corrosion that occurs in some cases, but in this application likely not to be too big an issue.
But why use stainless mesh? That will surely be quite expensive? Galvanized mesh much lower cost, and then use galv. staples to secure?

A. Hi Mohamed. If there is any moisture at all, significant corrosion of the galvanized frame will occur. Try to electrically isolate the two materials with the rubber beading you mentioned or in some other way.

Regards,

Hi Joseph. If the installation is inside and in a dry area, probably not -- you can't have galvanic corrosion without moisture.

But the question always involves the issue of the seriousness of error or miscalculation, because the fact is that the stainless steel and galvanizing are not galvanically compatible. Good luck.

Regards,

A. Hi Paul. Corrosion issues are of course complex. But if the cathode area is very large (the stainless cable) and the anode area is small (the staples), that accentuates the seriousness of the corrosion. Another factor is the copper in the pressure treated wood, which is also highly corrosive to the zinc of the galvanizing, and to the steel after the galvanizing is gone. These factors have to be added to a consideration of the environment (wetness and saltiness) and a consideration of the seriousness of failure.

I don't know whether stainless steel staples are available, but if they are, that's probably what I'd be looking into. Good luck.

Regards,

A. Hi. It doesn't sound right to you, and it sounds completely ridiculous to me :-)

Probably the cabinet was passivated and the hardware was not, or the cabinet is of a higher grade of stainless than the assembly hardware.

By installing the hardware with steel tools, it has perhaps lost its passivation. You could either try to repassivate with a citric acid based passivation solution, or try to research if the hardware and cabinet are the same grade. I would guess that the cabinet is 316SS and the hardware is 303SS. Good luck.

Regards,

A. Stainless Steel is rusting. No!! That can't happen. Worst name ever for any alloy. CRES (Corrosion REsistant Steel) is a much better name. No metal, under the right conditions, is corrosion proof. Not even Gold.
I think you got it right Ted. Dissimilar alloys in a salt air environment would be the right conditions. Especially after low alloy steel tools were used to assemble the cabinets.

A. Hi Al. I think the stainless screws are the best answer for this situation. Their surface area is small compared to the surface area of the galvanized tubing.

Regards,

A. Hi Eileen. If stainless plates are unavailable or too expensive, you should probably switch to galvanized fasteners for the galvanized plates.

Realistically though, it's probably not a big deal if you already have the stainless screws because, although the galvanizing will corrode to protect the screws, the screws are presumably small compared to the plates.

Regards,

A. Hi Shajahan. It doesn't sound like a good idea to skip the "rubber lining". The surface area of the pipe is much greater than the hangers, so the galvanizing will probably corrode away quickly if there is any conductive moisture. Also, see the comments regarding zinc embrittlement of stainless steel. It doesn't sound difficult to insulate between the pipe and the hangers though.

Regards,

A. Hi John. Ideally, there shouldn't be a metallic connection between dissimilar metals, and that would solve the problem. But if that isn't possible, then it becomes a question of the severity of the overall circumstances.

- On the negative side is that frequent wetness, and corrosive and ionically conductive de-icers are probably to be expected on a bridge.
- On the positive side is that stainless steel tends to not carry high galvanic currents because its chromic oxide skin isn't conductive, and powder coated materials, except where they are scratched or broken, won't carry any.
- The unknown factors for me are the area of the stainless compared to the galvanized area (if low, then galvanic currents corroding the powder coated and galvanized surface are minimized), and the criticality of corrosion (is the bridge structure at risk, or is there any other realistic threat to life?)

Regards,

A. Hi David. Although I acknowledge that plating & galvanizing can peel if poorly done, I've personally never witnessed the galvanizing flaking or spalling off a nut or bolt in the field in my own experience. Hopefully someone who galvanizes such hardware will reply with some statistics or inside info. Yes, passivated 316SS is ideal, but if a little surface rust/discoloration isn't a big issue, 304 will probably do.

Regards,

A. Hi Steve. I'm just the site curator with some limited personal experience in a few topics, not a bulkhead builder. But my experience with my own bulkhead and deck on a salt water lagoon is that the new pressure treated wood (ACQ) is so corrosive to zinc and steel due to its high copper content, that any additional corrosive effect caused by stainless steel is probably roundoff error not to be fretted over. I don't see any upside to stainless though :-)

BYou may find that your stainless nuts won't fit galvanized threads, which are much bigger to allow for the galvanizing thickness.

Regards,

A. Hi John. I see some concerns expressed above regarding issues like "liquid embrittlement" and explosion hazard" which I am unfamiliar with but which sound exaggerated or remote for your particular situation, but you might check building codes to see if there is any prohibition of stainless fasteners with galvanized ceilings. I think galvanized or at least zinc plated or painted drywall screws would be a better idea.

Regards,

A. Yes there are issues in using dis-similar metals in contact with each other in such an environment.
You could use insulating washers to insulate the dis-similar metals. The issue isn't proximity, but contact as in electrical contact. Isolate them from each other (plastic washers or similar) and the problem is avoided.

A. It is possible to double dip your product to get a thicker coating of galvanization for a little more protection against corrosion.

A. For this purpose I would not worry about the issue. When you use your carriage on the beach, wash it with fresh water when you get home to reduce the salt.
In theory the SS rod inside the galv pipe will be a dissimilar metals galvanic corrosion issue, but probably not any more noticeable than the wear of a dry axle than is worth worrying about.
You could put in a grease nipple and add some grease!

A. Hi Edmond. Galvanized screws seem like a much better idea -- is there a reason you can't use galvanized fasteners? If you can't, is there a reason you can't use insulating bushings along with washers to prevent galvanized surfaces from contacting stainless ones? Between the potential for galvanic corrosion and the dangers in fire situations that Alabbasi mentions, stainless screws contacting galvanized decking sounds troublesome.

Regards,

  Thanks for the quick response! You confirmed our thoughts.