Rust in low humidity

[Kent Adams]

I maintain the humidity in my workshop below 30% at all times, yet, rust has developed on some of my tools that I didn't wax (Starrett squares and straight edges). Tools in question are less than 4 months old. Any theories? Its not the humidity monitor, as it has been professionally calibrated. Currently, its 96% humidity in the ambient area outside my workshop but only 28% inside the work shop. I run both a dehumidifier and A/C constantly and the shop is very sealed very well. My shop is 12 x 24. I've checked for micro-climates where the rusted tools have developed as well, so that isn't the answer either. I'm perplexed.

[Ellen Benkin]

I get rust on my tools and I'm in the desert of Los Angeles. I put containers of Damp Rid, which absorbs moisture, in my tool cabinets and that seems to help.

[Robert Engel]

All rust is due to condensation, so my thinking would start there.

I live in FL and right now the humidity ranges from 60-90%.
My shop is not climate controlled.
I keep my hand tools in a sealed cabinet with a tub of damprid.

I don't understand why are you keeping the humidity that low?
What is the ambient humidity in most homes out there?
I wouldn't want my wood any lower for than my home for a furniture project.


I don't have much problems with rust on my machines as long as I'm using them fairly frequently.

[Kent Adams]

All rust is due to condensation, so my thinking would start there.

I live in FL and right now the humidity ranges from 60-90%.
My shop is not climate controlled.
I keep my hand tools in a sealed cabinet with a tub of damprid.

I don't understand why are you keeping the humidity that low?
What is the ambient humidity in most homes out there?
I wouldn't want my wood any lower for than my home for a furniture project.


I don't have much problems with rust on my machines as long as I'm using them fairly frequently.

Robert, good question. Typically in the summer months, humidity is similar as Florida, 80's and 90's at night. Right now, its 98%. I keep wood in my finished basement where humidity runs a constant 55%. I'm keeping humidity low in my shop to take some of the strain off the air conditioning, but also to aid in rust prevention. In my house, humidity runs about 65% during the summer, dipping a bit during the day.

[Bob Varney]

Could it be from salt from your hands?
I have worked at a retail store for woodworking, always have problems with rust on tablesaw tops in the form of finger prints, it was air cond.

Bob

[Ted Reischl]

As Robert wrote "all rust is due to condensation".

I do not live far from you, near Wilmington, NC. Our humidity runs about the same as yours (I used a machete to slice my way out to the shop this morning). My shop is also a similar size to yours 14 X 28 with 9' ceilings. It is probably not as tightly sealed.

I have zero rust problems. None, nada. I use an 11,000 BTU unit for cooling, it is also a heater for in the winter months.

So how come your stuff is rusting and mine is not? I am thinking that one thing I do is keep the air moving in my shop. There is a small fan that I let run pretty much continuously to circulate air when I am not in the shop. By keeping the air moving around the tools and machines it keeps them at the same ambient temperature as the air. Condensation (rust) forms on metal when the metal is much colder than the surrounding air. It does not matter if your relative humidity is only 30%. There is still moisture in the air and it will condense on colder surfaces.

I am curious about your statement concerning running a dehumidifier to take the load off the A/C? Most dehumidifiers actually warm a room up. The A/C units do not sense moisture to turn themselves on. The A/C unit will draw the same amps whether or not the humidity is low or not. So you are not "taking the strain off" the A/C. Instead, you are warming up the air and causing the A/C to run more often.

Starrett tools. . . .great tools, no doubt. Over the years I have noticed that they seem to be most rust prone tool in creation. Most of their tools go into machine shops with a nice oily atmosphere which is probably why they do not rust in the machine shops.

[Kent Adams]

I hadn't thought of that scenario but very possible.

[David Hawxhurst]

AC's work best at 50% RH or less. for health and prevention of mold RH in your house should be 40-45% for those of us that live in high humidity, lower is okay to. my shop/garage is ac and dehumidifier set at 45% no rust on my tools, but i generally wipe them down with crc 3-36. my shop experiences so pretty severe humidity swings during the course of the day. when we go in/out the garage it will spike as high as 75-80% but will come back down to the high 40's fairly quickly. with the humidity under control i've notice that the ac doesn't run as hard or long as it did before we put in the dehumidifier. also it is now much easier to prevent rust. our garage ac is a mini split with heat so it pretty much is moving air all the time as well as the humidifier has a fan that moves air as well.

[Kent Adams]

I have a very large older home with a new 5 ton multi-stage HVAC with a dehumidifier and ultraviolet filter along with a hepa primary filter. There is no way of getting humidity down below 50% during the summer in my house. My workshop is a separate building and easy to control the humidity.

[David Hawxhurst]

our house just to run in the mid 60% range, but we added a large heavy duty dehumidifier and sealed things up and not it runs around 45%. few people understand the need for hepa filters or uv light. i don't use or believe in using the uv light (based on my research). sealing up the house better would cause the humidity to decrease some. if you need it to go lower the a bigger or additional dehumidifier(s) would be needed. our problem was mainly coming from below so a full vapor barrier was needed along with sump pumps to catch the water coming from below/around the foundation. that wasn't enough so we added a dehumidifier rated for ~2000 sq ft. that was able to get below down to around 40% which translates into around 45% in the living area. in the change over from ac to heat the humidity gets to around 50%. we have enough water vapor from below that we don't get static electricity in the winter while heating the house. the dehumidifiers run year round. in my opinion vapor barriers in the wall should be the minimum code but its not. so guess whats not in my walls. also the foundation and basement slabs should also have vapor barrier as minimum code, again not here.

[Kent Adams]

I appreciate your guidance on my home's humidity remediation, but I'm asking for feedback in regards to my shop environment, which is a separate building.

[Kent Adams]

As Robert wrote "all rust is due to condensation".

I do not live far from you, near Wilmington, NC. Our humidity runs about the same as yours (I used a machete to slice my way out to the shop this morning). My shop is also a similar size to yours 14 X 28 with 9' ceilings. It is probably not as tightly sealed.

I have zero rust problems. None, nada. I use an 11,000 BTU unit for cooling, it is also a heater for in the winter months.

So how come your stuff is rusting and mine is not? I am thinking that one thing I do is keep the air moving in my shop. There is a small fan that I let run pretty much continuously to circulate air when I am not in the shop. By keeping the air moving around the tools and machines it keeps them at the same ambient temperature as the air. Condensation (rust) forms on metal when the metal is much colder than the surrounding air. It does not matter if your relative humidity is only 30%. There is still moisture in the air and it will condense on colder surfaces.

I am curious about your statement concerning running a dehumidifier to take the load off the A/C? Most dehumidifiers actually warm a room up. The A/C units do not sense moisture to turn themselves on. The A/C unit will draw the same amps whether or not the humidity is low or not. So you are not "taking the strain off" the A/C. Instead, you are warming up the air and causing the A/C to run more often.

Starrett tools. . . .great tools, no doubt. Over the years I have noticed that they seem to be most rust prone tool in creation. Most of their tools go into machine shops with a nice oily atmosphere which is probably why they do not rust in the machine shops.

Robert, I too run a 11k BTU A/C, its a window unit and I have a separate direct vent Rinai natural gas heater in the winter.

You're correct that the dehumidifer is producing heat. I'm operating off the belief that the dehumidifier lessens the strain on the AC having to lower the humidity. I don't know what the humidity is with just the AC on, but its worth me giving it a go.

I like your idea on running a small fan in the shop and I might try that.

[michael langman]

Working in machine tools my whole life I saw rust problems mostly caused by the acid, salt in peoples hands. I always keep a clean rag soaked with wd40 in my toolbox, and clean all of my hand tools off after each use.
You describe rust on hand tools so I think the above may be your problem.

[Kent Adams]

Thanks Michael. Looks like I will need to spread some rust preventative on the tools

[Mel Fulks]

It's been mentioned here that things like muriatic acid can cause problems even when the top is on. If you have some,store it somewhere out of the shop. Anti rust "emitters" are effective and available in different forms. They actually move toward steel surfaces and leave an invisible and oil less film.

[Andrew Pitonyak]

Thanks Michael. Looks like I will need to spread some rust preventative on the tools

Q quick wipe down after use for the day helps, especially if there is a bit of oil of some sort on the rag.

[ian maybury]

Direct causes aside the factor that determines whether or not moisture condenses out on a surface is its temperature - versus the dew point temperature of the surrounding air. If the surface is cooler than the dewpoint then condensation is inevitable. 30% RH sounds very dry, but there's still lots of moisture in the air at that. It's surprising, but if the room temperature is also pretty high (maybe 75 - 80deg F), then a surface doesn't have to be all that cold to cause condensation - maybe below 50 deg F.

Two other posible contributions. Your outside air is very humid and probably very warm, so if opening a door or whatever (or just some sort of tiny pressure difference caused by even wind outside) causes decent volumes of it to move into the shop then it'll probably overhelm any dehumidifier and condense like mad. Or if mixing/circulation is low it may condense locally. The humidity meter might not be picking this up if it's either fairly transitory, or it's in the wrong location.

I'm not hugely familiar with domestic dehumidifiers, but if 96%RH outside air is dropped to the required 30% RH at say 75deg F room temperature then the machine (presuming it's a refrigeration type/depending on the final room temperature required) is going to need to drop it down to something below 40 - 50 deg F to get enough water out to achieve this. Which cold air if it is blown out directly on to tools (without being fully re-heated to the room temperature because maybe it's cooling the room/offsetting heat gain from outside as well) could (?) cool them enough to cause problems. Depending on how it responds to leakage of high humidity outside air into the room it might even need to be drying (= cooling) the air quite a bit more than that to keep the room at 30% RH average, or as seen by the sensor.

There's always a risk of condensation when there's potentially rapid short term fluctuations in room/air temperatures, even more so if there's the potential for very humid air to be around as well. There's actually much less risk of condensation in relatively high humidity air (that's reliably - if only by a small amount - below the dew point) when temperatures and humidities are stable, than in much dryer air when temperatures are bouncing about - or when there's also a risk of a blast of warm wet air from elsewhere.

Heavy chunks of metal worsen the position, because when there's a sudden rise in air temperature it takes ages for the metal to get back to the air temperature. e.g. your tools get cooled right down, and then a mass of warm sticky air comes in from outside. Hose pipe job....

PS Sorry Kent. In direct words: there may be other factors in play, but (1) Low % RH doesn't necessarily protect against condensation if there's cold chunks of metal or pockets/streams of cold air about - even locally. (2) High humidity outside air coming in needs to be minimised, and quickly well mixed with the room air. (3) If the dehumidifier is blowing cool air on a cooling cycle it shouldn't aim at/cause extra cooling of tools - output air needs to be quickly mixed with the room air. (4) Aim to maintain a stable relative humidity and temperature in the room. (it neeeds adequate equipment of sufficient capacity to maintain control and ensure rapid mixing as above)

[John Lankers]

Kent, I think Ian is on the right track here.