Bolting Stainless Steel to Mild Steel

If you does not provide an electrical insulation between the stainless steel and the mild steel, you'll face galvanic corrosion in the mild steel.

Do not forget also to isolate the bolts/nuts or washers at least in the areas it contacts one of the joint elements.

thanks for that, what is the best material to use as the seperation layer?

If your operating environment is not too agressive, painting the surfaces that are going to mate is enough. Of course, any painting with metallic particles should be avoided. It is common to use a good primer to make such protection.

there are various types of corrosion. how can we understand this is the one. general corrosion galvanic corrosion concentration cell corrosion pitting corrossion intergranular corrossion stress corrosion cracking dealloying errosion corrosion microbially induced corrosion

"there are various types of corrosion. how can we understand this is the one."

By definition, galvanic corrosion is the corrosion that exists when two dissimilar metals are placed into contact. The various modes of corroding, (pitting, intergranular etc. describe ways that things corrode, not all of them are related to galvanic.

Galvanic tells you that by definition bad sh*t is going to happen when you put two dissimilar metals into contact.

if you have two different metals touching- steel and aluminum, steel and stainless steel, copper and iron, etc etc etc, the chemical activity is inevitable unless preventive measures are taken.

milo

Do a simple experiment yourself- put a penny on a piece of steel and moisten it look at it over nite; or put a piece of steel wool on a n expensive stainless steel appliance to get a more costly and permanent lesson. galvanic is why we never use steel wool on stainlesssteel.

milo

All good answers here.

milo

Hi Milo

We had the same problem with stainless steel hydraulic tube fitting to normal fittings. For years we used a product called Duralac made by Llewellyn Ryland Ltd. We then found that it was carcinogenic !

I'm still alive and I still (secretly) use it!

I'd first try to find some supplier source for industry specific insulating washers already manufactured for the application. I have to believe there are other applications similar to yours in some area of the construction industry. But if I couldn't find what needed that way I'd go to a gasket fabricator and have him cut me some 5 mil thick mylar washers with an outside diameter about 4-5 times the bolt diameter. If I were in some remote area where mylar sheet wasn't available I'd substitute 10 or 15 mil PVC sheet or the type used to make heavy duty industrial tarps and maybe have an available worker punch them out with gasket punches and/or gasket circle cutting tools.

Check McMaster-Carr for tools like this. They sell internationally through the internet, have an outstanding on-line catalog and accept major credit cards as well as more traditional payment methods. BTW, this company should be in every engineer's tool box. Their service is outstanding especially when you need something that local hardware suppliers do not carry.

Ed Weldon

Does anyone have any knowledge of the affects of plating the mild steel parts?

The plating of the mild steel will make it work as if it were made from the plating material. You may help a little by reducing the anode/catode differential with the plating, but depending on the plating material you may even increase it, or, even, to cause the stainless steel to corrode instead of the mild steel. And it's an expensive way...

Against galvanic corrosion, always try to isolate parts first.

Thanks, this supports my own thoughts.

We deal with a lot of different fluid conveyance fitting materials and surface finishes. Many times carbon steel, aluminum and stainless steel are mixed in assemblies. It's rather common. And, to my surprise, there doesn't seem to be many issues with corrosion.

In my own designs I avoid mixing materials as a rule.

Comment by gdevine:

"We deal with a lot of different fluid conveyance fitting materials and surface finishes. Many times carbon steel, aluminum and stainless steel are mixed in assemblies. It's rather common. And, to my surprise, there doesn't seem to be many issues with corrosion."

I think the extent to which there will be corrosion problems depends a lot on the presence of liquid electrolytes. Any time you get moisture, high humidity, high temperatures and acid or salty materials together you will get problems. This is a serious problem in large wooden sailboats (a bit of a rare bird these days) used in ocean environments where structural metal elements are mixed metals rather than strictly silicon bronze. One example I recently read about had zinc anodes in large quantities and still had problems with wood structure deterioration until in a rebuild the boatbuilder used mylar sheet between the wood and steel structural components. This is what prompted my suggestion of special mylar washers.

The geographical location has a big influence. What may be no problem at all for an indoor location in Northern California can become a big maintenance problem outdoors in Alabama. Or even worse, a small problem you never hear about but poisons your future sales into an important market area. ("the maintenance crews hate that equipment; they have a terrible time getting the screws apart so they can make repairs")

Ed Weldon

Our assemblies see varying environments. The applications are aircraft and military land vehicles. Certainly some of the most extreem environments any vehicle will see.

The good news is that these vehilces have rather intensive maintenance schedules. So if corrosion does become an issue it should be caught early.

The bad news is that the vehicles usually see many decades of service. There is certainly sufficient time for corrosion to occur.

Would you have info as to what happens when you weld a SS bracket on a mild steel tank or structural steel member? Would corrosion occur at the weld seam. Would SS welds rods be a must?

Thanks

Slight possibility of hydrogen embrittlement. However it is more a concern on medium and higher carbon grades especially after heat treatment.

milo

One method used in the marine industry where you have a steel hull and aluminum super structure is to use a bimetalic strip between the two dissimilar metals. The bimetalic strip is explosively formed.

You still have to isolate the fasteners. Often overlooked is placing the bolt in a sleve (nylon, UHMW or the like). A plastic washer needs to go under the nut and bolt head as well.

Travis

It all depends on the environment! You will need more than paint or thin strips of teflon if the environment is aqueous. Remember that this is a joint, which implies that it is load bearing. If you put some sort of insulation between the two metal plates and the joining hardware it would compromise the joint. The best way to avoid this is to weld a stainless flange (plate) to the carbon steel structure so that the joint is stainless to stainless. Then one could better protect the weld joint using TSA and paints. This has a much better chance for long term survival. Remember to use the same type of stainless, because different alloys of stainless will act very much the same, just not as aggressively.