Rust Busters
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This article is from Issue 51 of Woodcraft Magazine.
A little science can help you pick the product that packs the hardest punch.
Rust needs no introduction. Woodworkers with unheated basement and garage shops are all too familiar with this uninvited “guest” that regularly stops by to snack on their shops’ contents. Those working in climate-controlled shops are likely to find rust blooms sprouting from the oils and acids left by fingerprints or born of condensation rings left by a casually-placed drink.
Whether restoring hand tools or reclaiming metal machinery, you have to arm yourself for the fight. The challenge is finding the right weapon. To help you choose, I gathered the top rust busters in the market and conducted my own side-by-side test.
The test revealed that no product is a perfect weapon. Selecting the right product starts by reading the label and understanding some of the science behind a few key rust-busting ingredients. Armed with this knowledge, you’re likely to select a few different strategies. Let’s face it, rust will eventually win the war. But after reading this, you’ll have what it takes to win a few battles.
About The Test
Against every woodworking instinct, I invited rust into my workshop. To do this, I first used acetone to remove any oils or protective coatings from my test tools, and then dipped them into salt water and set them aside to deteriorate.
Two weeks later, I attempted to undo the damage. I treated the tools as specified by each product’s instructions, and periodically inspected the surfaces until the rust was gone. After a neutralizing rinse, I lightly scrubbed the tools with a non-abrasive pad to knock off loose residue.
Choose your weapon
Despite the multitude of products, rust removers can be categorized according to a few key ingredients. Within a category, each treatment works in the same basic manner.
After a quick acid dip, rust revisited a cleaned tool. A longer bath resulted in a protective coating.
Selective chelants remove rust and produce a protective patina, but can compromise good metal.
Electrolysis won’t replace missing metal. Pitted chisels and blades require further smoothing.
Acids
Nearly any acid can remove rust. The process can take from a few minutes to several hours, depending on the acid’s strength and the amount of rust.
Phosphoric acid is the fastest acting, but the least forgiving, especially when cleaning tools with either painted parts or components made from various metals. Phosphoric acid, in liquid or jelly form, is not recommended for use on painted steel, chrome, or even stainless steel. And these acids will quickly dissolve aluminum components.
Acid does not differentiate between rust and good metal. Even milder acidic solutions will etch sound steel. (Under a microscope, etching resembles millions of tiny pits–each a fast-food restaurant for future rust.) Unless dried and oiled immediately after treatment, freshly cleaned surfaces will begin to “flash-rust” within minutes. (See photo above, left).
Phosphoric acid was the only exception to the flash-rusting rule. Leaving a plane blade in phosphoric acid for an hour produced a black, fairly uniform coating of ferric phosphate. This coating can prevent corrosion, but it’ s not as reliable or convenient as other corrosion inhibitors. (See “Keeping Rust at Bay” on page 35.)
The greatest dangers relating to acid are to the user. Phosphoric acid can cause burns or blindness upon contact, so gloves and goggles are a must. Kitchen-grade products containing oxalic and citric acids can be used without gloves, but prolonged exposure to bare skin will leave your hands raw.
Selective chelants
Chelants are compounds designed to steal the iron from iron oxide, inactivating it and holding it in solution so that the resulting residue can be rinsed off. This selective process does not affect steel, copper, brass, aluminum, plastic or rubber. All the products listed are non-acidic, water-soluble, biodegradable, and safe to handle without gloves. (The manufacturers don’t divulge the ingredients. I grouped the products together because they perform similarly.)
Unlike the acids, chelants don’t require rinsing; in fact, remaining residue offers some protection (see photo above, center). The solution can be reused, but in time it will leave a darker deposit on your derusted tools. (I save the used stuff for heavy removal and employ a fresh batch for final cleanup.)
Although safer for you and your tools, chelants don’t always discriminate between good and bad metal. In a few instances, the chelants selected sound steel, etching into the lamination boundary lines on my older plane blades, and even creating pits that required grinding.
Electrolysis
This process uses electrical current to convert red rust to black. All you need is an energy source (battery charger), a sacrificial anode (iron scrap), a dip tank, and a tablespoon of baking soda to get the current flowing. Attach the negative lead to the tool, the positive lead to the anode, and plug in the charger. (You can’t see electrons making the jump, but bubbles indicate that the reaction is underway.)
Of the options addressed in this article, electrolysis requires the greatest initial expense. However, once you’ve made the investment, that’s it. And, unlike the other chemical treatments, this plug-powered process is not temperature-sensitive, which is a plus when working in an unheated shop.
Unfortunately, electrolysis isn’t suitable for tools with non-removable wooden or electronic parts. And despite some claims to the contrary, it doesn’t “reverse rust”. Lost steel is lost. When restoring plane and chisel blades, you’ll need to grind or hone any pits down to clean steel (see photo on page 33).
We Have Met The Enemy, And He Is Rust...
There are two forms of iron oxide–one red and one black. It’s the reddish version, Fe203, that you need to worry about. Because red iron oxide is larger than the surrounding molecules, it causes the affected steel to “puff,” creating cracks and voids that expose more bare metal.
Unlike red iron oxide molecules, those in black iron oxide (Fe204) are the same size as neighboring metal molecules. These oxidized areas will not grow or flake. Black iron oxide is safe; in fact, it’s what makes up most of the patina on old tools. Some hardware is deliberately treated with black oxide to keep red oxide from getting a toehold.
Keeping Rust At Bay
In order to oxidize, iron must be exposed to oxygen and water. Lathering tools in Cosmoline or relocating your shop to the moon might work, but neither solution is terribly practical. The best defense requires a combination of products.
Moisture-displacing coatings provide decent front-line defense. In my informal tests, the products shown above helped abate rust after the derusting treatments. In addition, these products coexist with the adhesives and finishes I use in my shop.
When coatings fail (and they will), you need a reliable second line of defense. Room-sized dehumidifiers work in basements, but in my garage the protection disappears the instant I lift the door. Instead, I rely on small-space sentries. Designed for closed compartments such as cabinets and drawers, dessicants absorb moisture before it reaches metal surfaces. Select a model that provides a clear indication that its working (many have color-changing crystals), and that can be easily recharged when it’s saturated. The least expensive models can be reactivated by heating them in an oven. Some have built-in heating elements so that they can be recharged without baking.
If you have access to a power source, a low-temperature heating rod will keep air circulating so that moisture can’t condense on metal surfaces. Originally designed to provide constant climate control in gun safes, rods are available in different sizes to suit larger spaces, such as tool cabinets.
Summary
Phosphoric acid-based cleaners may rank as some of the fastest rust busters, but they’re not much quicker than less caustic acids. There’s no shame in sticking with citric and oxalic acids, especially if you prefer keeping products bearing skulls and crossbones out of your shop. Bar Keeper’s Friend remains my favorite for light rust treatments and final polishing, but I plan to keep a small bottle of Rust Free for spot repairs.
Selective chelants cost more than acids, but prove safer for both skin and steel. They provide the best solution for the widest assortment of tools, especially those with non-detachable, non-ferrous parts, or those difficult to disassemble and oil afterward. As long as you keep your eye on the clock to prevent unwanted etching, these products offer the best convenience.
Electrolysis requires more time and money, as well as a tank large enough to submerge your work. Despite these obstacles, its ability to attack rust while leaving steel unscathed makes it the best method for prized antiques.
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