@JimT: Yes, stainless steel would work in this application. If you're ever in doubt about what will or won't corrode in a given situation, the galvanic series is a good reference. The most noble (cathodic) metals are at the top of the list, and the least noble (anodic) metals are listed at the bottom.
Corrosion is an electrochemical reaction. For corrosion to take place, you need three things: an anode, a cathode, and an electrolyte (usually water). For any combination of two metals, the less noble of the two will be the anode, and the more noble of the two will be the cathode. The anode will corrode; the cathode will not.
That's why cadmium and zinc make good coatings for steel. As you can see, they are both less noble - lower on the list - than steel. They act as sacrificial anodes, protecting the steel from corrosion until they are eventually used up. If you're familiar with boating, they perform exactly the same role as the zinc or aluminum anodes on an outboard engine.
In this case, the wood has been treated with copper. Copper is significantly more noble than cadmium, so the copper will be the cathode, while the cadmium will be the anode. Wood is typically porous, so there is plenty of opportunity for electrolyte (i.e. water) to circulate.
Corrosion rates are related to the anode-to-cathode surface area ratio. The smaller the anode relative to the cathode, the more quickly it will corrode. In this case, the anode - the cadmium coating on the bolts - is very small relative to the cathode - the copper-treated wood. This will lead to relatively rapid corrosion.
Another look at the galvanic series will show you that copper is also more noble than steel. This means that, once the cadmium plating is used up, the steel will begin to corrode. This explains why the bolts were "eaten up."
Looking again at the galvanic series, you can see that 300-series stainless steel is more noble - higher on the list - than copper. This means that stainless steel will not corrode in this situation. That's why it would be a good choice.
Its surprising to me that the various treated bolts all rotted away. The set was originally supplied with Cadmium-Plated bolts?Surprised for two reasons; 1st that the cadmium failed (commonly found on aircraft parts) and 2nd because of the negative environmental press that Cadmium is getting these days ,,,, and it was selected for a children’s play set-? Then, to have the good old stand-bye for longevity; zinc galvanized bolts as the second string replacements. Most chain link fence hardware is fastened with galvanized zinc and those fences last for decades. The point is clear it’s the wood doing the damage to the bolts, which seems counter-intuitive to me. But just read over some of these other posts by persons who have extensive experiences with lumber and corrosion and it seems like they saw this one coming. Just one more reason why a well rounded background, (in this case chemistry coupled with mechanics) is increasingly important for good product design.Wonder if Stainless Steel bolts would have lasted ?
Even twenty years ago most hardware stores would steer you to the correct fasteners. Lowes and Home Depot have signs in the screw and nail section. The Building Departments in the last three counties I've lived in require Building Permits for Decks etc. and the paperwork specifically notes the use of appropriate fasteners for treated wood. Cast Iron and Aluminum rust/corrosion is caused by a "battery" reaction and the reaction of Copper being reduced accelerates this. Arsenate is also reduced and can form extremely toxic Arsine vapor when reduced even further (e.g. Zinc). The carbon in cast iron also creates a corrosion potential. If you live in a high rainfall area or near salt water the moisture and chloride ion also increases rusting. Cadmium is also cancer causing and unless the swing set was ordered directly from a foreign country the bolts probably were not cadmium plated. Sulfate, phosphate, Ammonia, Carbonate, et al also act to accellerate corrosion. Galvanized or Iron dipped in liquid zinc will only protect until the Zinc is depleted locally, as noted Zinc corrodes rapidly in the presence of salt and the generated acidity will further increase that. Hard Zinc plating, especially Zinc-Nickel or Zinc-Cobalt is less porous than Galvanizing. Note that there are many varieties of "Stainless Steel" and many do not resist corrosion in a non-aerobic environment e.g. underground.
A newer treatments for wood are Zinc Borate, Borate alone, or Sodium Silicate (WaterGlass). (Borate is almost non-toxic: boric acid is used in eye drops and Borax is a food condiment in the Middle East.)(Water Glass is used to preserve boiled eggs.) These treatments don't have a corrosive effect on fasteners.
Doesn't it seem a bit strange this information would not be front-of-mind with the manufacturers of this equipment. If not initially, in time, I would think customers would alert the manufacturers to these defects. Many of the customers are likely to be public parks, where the customer has an ongoing relationship with the manufacturer. I would think this would come to the manufacturer's attention.
The U.S. Army uses copper naphthenate to preserve oak planks used to line the beds of large trucks and trailers. Telephone poles are also treated with this material. It is effective against insects and fungus, but not very harmful to humans. It also makes the wood more splinter resistant, and does not interfere with weatherproofing materials such as Thompson's Water Seal. Avoid using copper naphthenate that has been thinned out with Diesel fuel because it is less effective and makes the wood more flammable. The Army recommends mixing 3 parts (by volume) copper-naphthenate concentrate (8% copper) with 11 parts standard mineral spirits. The mineral spirits help the copper naphthenate soak into the wood, then evaporate, making the wood more flame resistant.
We have been using standard zinc-plated screws in wood treated with copper naphthenate. Copper naphthenate is not a salt and is not soluble in water, so I don't think it would cause corrosion, but does anyone know if actual corrosion tests have been done, perhaps by UL?
Materials compatability is something all design engineers need to think about carefully. For example, you might want to think twice before using zinc-plated hardware with nylon or polyacetal in a marine environment. As the zinc corrodes, it forms zinc chloride, which will attack the plastic. The same thing can also happen with brass. First, the zinc de-alloys from the copper; then, the zinc forms zinc chloride, which attacks the plastic. Nylon and polyacetal are both known for their chemical resistance, but as it happens, zinc chloride is one of the few chemicals they aren't resistant to. As a materials engineer, this is one of the many non-obvious incompatabilities which I have encountered. Just another thing to watch out for!
One item of note: the article never mentioned that creasote was used for playground equipment, it mentioned use as boat pilings.
Also, the article mentioned, but did not address, toxicity concerns, as the focus was safety of the mechanical structure of the playset.
The comments on CCA vs ACQ treated wood are well appreciated, since (if you can believe it) there was no description in the manual of which type of treatment was used. If ACQ looks and smells the sames as CCA, then I can believe that was what was used.
WE got around that by getting a metal play set that had stainless steel bolts. When it was time to retire the set I took out all of those bolts and saved them, since all that they had was some dust. Probably the stainless bolts would have been cheaper in the long run, and certainly they would have been much less work.
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