NAS D110 Rust - Solutions?

[JimC]

I'd be very interested in seeing a picture of a part that failed or was materially compromised due to the installation of a stainless fastener.

[J!m]

Just pick up a first year materials science book...

If you make batteries, you understand the release of energy comes from a chemical reaction.

The ions of the less noble metal are consumed to produce electricity (this is electrolytic corrosion).

When the similarly noble metal (steel) is in contact with aluminum, it is reasonably stable. The problem occurs as the steel begins to degrade. the iron oxide is an abrasive and begins to remove the outer oxide layer of the aluminum (anodize) and/or any paint and then begins to (physically) attack the aluminum. Add the electrolyte (salt water) and you have a nice battery (steel-aluminum). (There have been reports of actual voltage readings from Land Rovers)

If you go to stainless steel (which has a lot of nickel which is far more noble than either steel or aluminum) you now have the opposite problem: the bolts are more noble than the aluminum. So, add the electrolyte, and the aluminum begins to corrode. So, you protect the cheap bolt to sacrifice the expensive body parts.

The same happens with steel against stainless steel, but usually the paint helps a lot because the material hardness is similar, and does not deflect as the aluminum does when the bolts are tightened... However, when rust forms, this abrasive action cuts through the paint and allows the process to start.

Zinc is less noble than aluminum, so when it is in contact with aluminum in electrolyte, it is sacrificed, and not the aluminum. They are so close, that the process is essentially stopped, but not completely.

Standard bolts are zinc 'washed' so they have a thin coating of zinc. All the new bolts on the Land Rover are this type. Over time, the zinc is depleted, and then the steel is in contact with the aluminum, so they begin the corrosion process.

Cadmium plated bolts may seem like a good idea, and in some cases it is a good idea, but Cadmium is more noble than nickel, so you have the same problem. And you add the environmental issues associated with Cadmium (nasty stuff) so I still say Zinc washed is the way to go.

It really is quite simple. Just do a goggle search and you will have all your answers.

I also know of one truck that used clear mylar between all the mating surfaces of differing metals. That truck has not suffered any corrosion after over 10 years. If you keep the materials from making contact, the process will be stopped.

[JimC]

I'm not confused about the fundamental properties of materials, its just that I've seen plenty of stainless fasteners in plenty of applications on Rovers and have never seen a part damaged due to the use of stainless steel. My argument is that, practically speaking, in a non-marine application, a stainless fastener is a big improvement and your advice dramatically overstates the problem. If you are building a boat or something else that will be immersed in salt water, plan accordingly, but if you want to save yourself a lot of hassle with corrosion on your truck, use all the stainless you want (in non high-strenght applications of course)

[J!m]

Well, I'm glad you understand the concept, and yes, stainless steel is not suitable for use where structural members are joined. (not in the same diameters anyway- up-sizing can allow stainless to be used, and the grade of stainless can also help)

I will spare us all the babble on the merits and down sides of where and when to use grade 5 and grade 8; shear load versus tensile load etc. If you fabricate, you had better know this stuff. I'll leave it at that.

So for our theoretical example, I will look at the bolts and nuts, behind the rear wheel of an 88" series truck that fasten the tub to the rear cross member. These are subjected to impact abrasion as well as salt (liquid) spray and fog from salted roads (not a problem in the southern part of the united States, but bear with me). This occurs even if a mud flap is in place on an 88".

The abrasion removes the Zinc on the nut and/or bolt, and the steel begins to rust. The rust creeps under the nut, and is accelerated by the presence of the salt. The rust/abrasion removes the paint from the frame tab, and this rust creeps under the aluminum body. The paint comes off the body. The body begins to corrode. The salt accelerates the galvanic corrosion as stated earlier.

If this is stopped, washed, painted, oiled whatever to interrupt the process, it will be slowed. If salt is not used on your roads, this may be slowed (salt occurs naturally too). The process of galvanic corrosion will happen; the question is simply how long it takes. If proper isolation is in place (such as the mylar), it will be halted. If it is not isolated, and an electrolyte is introduced to the system, corrosion will take place, and quite fast in some cases. The addition of oil, washing, painting imparts a barrier and/or removal of the accelerate (the salt). the oil and paint blocks the air (oxygen) from free access to the open metal.

Mike has a 1965 series IIa that has nearly no corrosion on it. We took that truck to North Africa, and he had many problems with nuts rattling off. The reason was that there was no rust to hold the nuts and bolts together. Since there was no rust, there was no galvanic corrosion either. They are interconnected, and this is why the corrosion is always faster at the attachment points than other points of the body (many areas of his truck have no paint, and are smooth, clean aluminum, with no corrosion). His truck was used on a farm in an arid area. Very little road time, and stored in a well ventilated barn, so there was no moisture build-up, and no salt accelerating the process.

Take an aluminum bowl, and fill it with super-saturated salt solution. It could sit for years with no apparent corrosion forming.

Now, take the same bowl of solution and add a steel bolt. It will accelerate the process, but it will still be slower than on the truck. Why? Because there is very little oxygen available for the process to oxidize the aluminum and/or steel. When a truck is wet with salt-water, and then allowed to be exposed to air, the oxygen reacts (oxidizing) the metal. If there is no oxygen, there is no corrosion either. This is why steel ships can remain under the sea for a hundred years and not be completely dissolved... There is oxygen in the water (fish like it) but not as much as the 20% that makes up air... It also explains why stuff NEAR the ocean corrode faster than things IN the ocean. (Tide-line is an exception to this and beyond the already huge scope).

I have done several salt-fog corrosion tests, as well as read a 25-year long corrosion study done in actual seawater (several locations around the world) by my company, and can speak with a bit of knowledge and experience on this subject. Immersion is not the same as fog. Not by a long shot. A car is more like a fog test, than an immersion test. Both cause corrosion; however the fog test is more aggressive by far because of the availability of the oxygen to combine with the metal, forming the oxide and releasing the ions to produce electricity.

You can goggle "ASTM salt fog corrosion testing" for more info.