Wood: Enemy of Metal?

[kevin]

Recently I happened to run across an interesting article on Corrosion of Metals by Wood (the article s from the National Physical Laboratory in England).

As a person who enjoys woodworking as well as metal working, I often make tool holders (as in for storing on a shelf or in a drawer) out of wood. In addition, machine tools are frequently packed in wood storage boxes (still happens, though plastic is becoming more common). But the article above made me wonder if this is a good idea or not.

The article is primarily directed at the use of wood for shipping and warehousing containers, but I think it has some application to the shop as well.

A few excerpts from the article:

• Wood is a corrosive substance by nature, and can be made more corrosive by treatment given to it. Unlike most other corrosive substances, one of the corrosive chemicals in it, acetic acid, is volatile, and in an ill-ventilated space, wood can cause corrosion of metal nearby but not actually in contact.
• Kiln drying accelerates the production of free acetic acid in wood, but most of the acid does not have time to escape. Kiln dried wood is more acid and more immediately corrosive than air dried wood, though it contains less combined acid that can be set free in later years.
• Different woods have different acid content, some examples from a longer list:
  • Oak - high
  • Birch - fairly high
  • Spruce/elm - moderate
• Unfortunately, no determinations have been made of the actual concentrations of acetic acid vapour inside wooden boxes, but it is clear that corrosion is caused by only very small quantities.
• . . . even with well dried wood, a corroding humidity can be reached easily in a temperate climate, and all the time in a tropical one. If the wood becomes wetted, corrosive conditions can develop at any temperature, and be very severe at higher temperatures.
• Avoid fresh wood. Avoid kiln dried wood. Avoid damp wood. Keep wood in a dry atmosphere for as long as possible before use.
• The emission of acetic acid vapour from wood is retarded, but not stopped, by paint or lacquer applied to the wood. It must not be forgotten, however, that . . . paints . . . themselves emit corrosion vapours (formic and acetic acids). Choice of coating . . . preferably acrylics and two-pack epoxides and polyurethanes.

In my own experience, whenever I have something like a collet holder out of wood, I have always given it a polyurethane coating. I would expect that if there was a significant problem, I would see it where the tool directly contacts the wood, but I have not observed any unusual tarnishing or corrosion in this regard. I suspect the most important thing in this regard, as in general, is to keep the humidity low, but it is probably a good idea to avoid direct wood contact if possible, and if using wooden drawers make sure there is plenty of ventilation.

 

[darkzero]

When I first got this chip brush, the day I got it I left it on the mill table overnight. It left a stain on the table in the shape of the handle. Came out with scotchbrite but lesson learned.

That's the only time I've ever experienced it but most everything else I have that comes in contact the wood is stained/cleared.

 

[mikey]

Excellent post, Kevin. Thank you!

 

[benmychree]

I have never seen any tendency to rust in my Gerstner tool box, nor any precision tools in wooden boxes, Perhaps an issue in warehousing situations, but a tempest in a teapot on my world.

 

[francist]

Early in my conservation training we were given an example of how not to store ancient coins. A museum had a collection of Roman coins (I want to say of lead, or similar alloy) and had made carefully fitted trays to hold them for exhibit. The trays were made of oak, and fitted into sealed drawers (sealed as in airtight). Within a short time, curators were horrified to find the coins covered with white corrosion product resulting from the acid content in the oak.

Granted, museum conservation people tend to fuss about very small amounts with highly sensitive materials, and real-life tolerances can be somewhat different. However, if you've ever had occasion to turn woods like black walnut on the lathe, especially when it's freshly cut, you'll soon find your iron tool rests, bed ways, and even your fingers nicely purple or black from the reaction.

I store a lot of my tools in wooden drawers and have for years without any problems. But, I'm still conscious of the potential and especially so if there's chance for moisture or higher than normal humidity.

-frank

 

[hman]

Kevin -
Great post!

Couple thoughts here ...

I definitely avoid oak. Usually use plain ol' fir (as in a chunk of 2x4). An example would be a rack for R8 collets. After I've drilled the requisite holes, I saturate the wood with mineral oil or way oil. Haven't kept any wood tool holders in sealed containers, so haven't had any issues yet. Here in Arizona, moisture isn't much of a problem. But even when I lived in Oregon, I don't recall any issues.

Having been a chemist in a previous life, and from the information in your post, I'm tempted to try dipping any future tool holders in ammonia, then drying thoroughly before oiling.

As for Gerstner tool boxes, the drawer bottoms are made of metal. I've assumed it was for strength. But it may well be that this reduces corrosion. I'm sure that if a Gerstner EVER caused corrosion of a precision measuring instrument, there'd have been a holy stink raised!

 

[hman]

OOPS! Maybe my idea of ammonia treatment is useless. The article states that "Lime washing, which might be expected to absorb acetic acid vapours, has been found ineffective." Ah, well ...

I did notice that elm has a pH close to neutral. Might be worth considering, on this basis alone. Mahogany doesn' look too bad, either. Ina I seem to recall a number of instrument boxes made of mahogany.

Lots and lots of info in that article!!!!

 

[cbellanca]

Yes, wood has an acidic component. I coat on a regular basis all my bare metal wood working surfaces including my table saw, planner, joiner, band saw, drill press. lathe and mill bed with Johnson Paste Wax. Even with the wax coating I have had the most problem with treated lumber. Boards or saw dust from treated lumber Left over night will produce a hard to remove stain. The wax also provides protection with the normal room humidity.

 

[Robo_Pi]

CAUTION ----- This post is kind of off topic ----- CAUTION

This is a story of wood versus metal in a slightly different way;

There used to be an 8 inch in diameter limb at the top of this Oak tree.


One very calm sunny day it decided to fall off the tree. I was sitting out in the yard and heard it crack. There was absolutely no wind.

I looked up and saw the limb break off the tree and begin it's 60 foot journey to the forest floor.

There was nothing that could be done to stop this limb from falling to the ground.

The problem was that my truck was parked directly under this limb!

All I could do was watch the event unfold before my very eyes.

I saw the rear window of the truck cap exploded into a frothy white sea of tiny glass particles.

I didn't think to take a picture while the branch was still on the truck. It was a Y-branch and the smaller part of the Y was about 6 inches in diameter.

Miraculously the side windows survived. And the windshield only cracked in the upper right corner on the passenger side. So the major damage was to the obliterated rear window.



I actually laughed. What else could I do? Getting upset about it wouldn't solve a darn thing. At that instant I just resigned myself to having one more unwanted project on my to-do list.

So with great patience and calm, I got the chainsaw out and cut the branch up to remove it from the truck. It was way too heavy to even budge in once piece. It wasn't a dead branch. It was perfectly live. I have no idea what made it fall off the tree. It must have been struck by lightening at one point is all I can figure.

In any case, I put the branch on the sawmill and cut it into lumber. Then I used some floor jacks inside the cab to jack the roof back up. Did a little hammer and dolly work. And then set about building a new rear window frame using the oak from the very branch that smashed in the roof.

Here's the final repair job. Now I call this truck my "woodie".

 

[silence dogood]

I read the article about the corrosion of metal by wood. However, there is a lot more to this. Salt based chemicals are used as a preservative for wood. That's why you use special coated fasteners and metal when you buy treated lumber. And yes there is acetic acid in oak. But there is also a lot of tannic acid in oak, which is used as a preservative for iron. The metal was also put in a sealed box for testing purposes, hmm. The wooden tool boxes use drawers so the wood can breath. Last, my house is made out of either fir or pine 2by's and sheathed with cedar ship lap fastened together with steel nails. My house should be a pile of rotting lumber with brown stains. Funny, it's still in one piece after 70+ years. I make wood handles and wood boxes for my metal tools. Never had a problem. Come to think about it. I have tools and knives with wood handles that are over 50 years old. The metal and wood are just fine. Sorry about overdoing my 3 cents worth.