Thread: Dissimilar Metals on seawall tieback rods

Inspected a waterfront house with a 1 1/2 year PVC seawall that has stainless steel tieback rods with a galvanized plate where the rod and nut is secured to the piling. The rods have serve corrosion between the wall and piling. The seawall contractor has determined that the water has stray voltage of 200 milli-volts DC power, which has caused the corrosion. While I don't disagree, in my opinion the contractor has caused the corrosion when he used dissimilar metals.

AGA American Galvanizers Association-website

'Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a binetallic couple because of the diffenent affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals.'

Would appreciate your opinions.

Similar Threads:

• connection of dissimilar metals
• buried ground rods
• Ground Rods
• Lightning rods on condenser units
• mixed metals

While that is correct, did you look at the table on their web site? I have this bookmarked from years ago: Dissimilar Metals in Contact with Hot-Dip Galvanized Steel

The tops are the columns on the table are 'Metal in Contact' - 'Atmospheric' - 'Immersed', and under 'Immersed' is 'Fresh Water' and 'Sea Water', the last column is what you want.

In the first column, under 'Metal in Contact' go down to 'Stainless Steel' (there are two types, 4th and 5th from the bottom of the table), go across to the last column and you will see '1 to 2', now go down to the 'Key' and you will see that '1' would be okay, but '2' would not be okay under some conditions. Sounds like you are describing '2' down in the Key.

Now go back up the chart to Chromium and across to the right column, that shows a '2 to 3' in sea water.

If the stainless steel is high in Chromium content, say 18% and up, then the reaction will be closer to the '2 to 3' for Chromium than the '1 to 2' for Stainless Steel. And if the Stainless Steel were to be between 13% and 18% Chromium, then that may be the cause of getting the '2' level of corrosion. They may have thought that using a higher quality (greater Chromium content) of Stainless Steel was better, but based on that chart it creates a worse condition for corrosion.

Okay, that is about the limit of my knowledge of what I have on that, and there will be some here with great knowledge on that (and they may say I am way off track, so I will learn too).

I would to try and locate the source of the stray voltage, it is dangerous. I hope you use plenty of zincs on and around your boats. Another good practice is to keep a zinc on a cable to each engine block, and throw it over when docked.

We are always building docks: all we use is galvanized. Even still, if there is excessive stray current in the water, we still lose pins. Its all part of maintenance.

Zinc is the sacrificial anode material of choice, no? So it stands to reason that zinc is eaten away fairly quickly, exposing the steel. The steel plates are rusting. I agree that SS plates would be better but very costly. I agree with Steve, that regular maintenance is a fact of life when you are dealing with docks in salt water, especially when there is electrical power supplied to that dock. How about a plastic washer under the SS nut? Might help.

Stainless steel is interesting, non-magnetic, and it does rust if it is stressed in some way, such as if it has been cut with a torch.

Originally Posted by John Kogel

Stainless steel is interesting, non-magnetic, and it does rust ...

Which is why it is called stain-less not stain-free

Originally Posted by Steven Turetsky

Which is why it is called stain-less not stain-free

I watched a show on the making of stainless steel a couple of years ago and the protection goes up sort of in relation to the percentage of chromium added to the steel, up to 18%, beyond 18% you can keep raising the percentage of chromium but there is very little gain in added protection, such as the difference protective qualities between 18% and 20% and 24% is very little, yet the cost difference is quite a bit - the main thing one gains by adding more chromium is cost ... ... that is what they stressed several times. Stainless steel is an alloy and the percentages of the other metals, as well as the types of other metals, added to the steel can dramatically improve the properties of the stainless steel or they can be detrimental to the properties of it - all depends on what one is seeking and willing to give up for something else.

I was suprised how quickly the tiebacks completely rusted, you think stainless is longer lasting than 1 1/2 years. A few months ago a marine contractor said he quit using stainless because certain soils were causing the rods to disintegrate. He was finding the tieback damage on the land side.

The wall I inspected was damaged on the water side primarily inside the piling. The marine contractor, who installed this wall, is blaming the damage on the stray current, which he says traveled from the water up the piling.

Could the chemicals in the piling cause the damage with or without current? Or is the current the main culprit, which is triggering a reaction to piling chemicals or dissimilar metals?

Worst yet, could this be Chinese stainless steel?

Stray current in the water can and will cause a great deal of damage.

Originally Posted by Steven Turetsky

Stray current in the water can and will cause a great deal of damage.

As well as loss of life, if not from the electrical shock from the drowning resulting from that electrical shock.

Additionally, as you pointed out in an earlier post, if cathodic protection if not provided for boats, then there can be severe deterioration and loss of metal components, propellers, shafts, outboard drives, you name it, especially aluminum parts and equipment.

They found DC stray current. Could be coming from boats tied to the dock?

Originally Posted by John Kogel

They found DC stray current. Could be coming from boats tied to the dock?

The most common cause is from electrical cords or cables submerged in the water. Could be from a shore power dock line(s), or a nearby factory, or something in between.

Does electrical current damage stainless steel?

Originally Posted by Jack Wingo

Does electrical current damage stainless steel?

The electrical current does not damage the stainless steel, it is the corrosion which can cause the damage, yes. And, yes, electrical current can make the corrosion worse than it would have been without any electrical current.

Originally Posted by Jack Wingo

I was suprised how quickly the tiebacks completely rusted, you think stainless is longer lasting than 1 1/2 years. A few months ago a marine contractor said he quit using stainless because certain soils were causing the rods to disintegrate. He was finding the tieback damage on the land side.

The wall I inspected was damaged on the water side primarily inside the piling. The marine contractor, who installed this wall, is blaming the damage on the stray current, which he says traveled from the water up the piling.

Could the chemicals in the piling cause the damage with or without current? Or is the current the main culprit, which is triggering a reaction to piling chemicals or dissimilar metals?

Worst yet, could this be Chinese stainless steel?

I am suspect of anything from China. They don't seem to worry much about following standards.

The type of SS also matter. what type are they using.

"Type 304 and 316 stainless steel have been used to provide maximum corrosion resistance. Type 304 and 316 stainless steel connectors and fasteners have been used in demanding applications such as coastal exposures and in permanent wood foundations."

Originally Posted by Jack Wingo

I was suprised how quickly the tiebacks completely rusted, you think stainless is longer lasting than 1 1/2 years. A few months ago a marine contractor said he quit using stainless because certain soils were causing the rods to disintegrate. He was finding the tieback damage on the land side.....?

Were there any other installations near this location, done by another marine contractor, that you could look at to compare the materials used and the results from exposure?