Passivating Stainless

Stainless steel is a miracle for boaters. It’s reasonably strong, stays shiny, and requires minimal maintenance. But savvy boaters also know it’s not corrosion-proof, only corrosion resistant (see “Marine Metal Warning”). In fact, for many applications it is inferior to silicon bronze, the copper alloy that has served mariners for decades, (see “A Look at Stainless Steel vs. Silicon Bronze” within “Mainsail Halyard Shackles“).

To ensure you get the maximum life out of your stainless steel, it helps to understand how stainless steel material resists corrosion and what we can do to protect it. In this report, we’ll compare several products that can be used to passivate stainless steel, a process that effectively stops surface corrosion in its tracks and offers longer-lasting protection than any wax or polish.

Because stainless steel contains high levels of chrome and nickel, instead of rapidly rusting in seawater like plain carbon steel will do, it forms a very tough oxide coating that is virtually corrosion proof. But this is only the case when the material isolated from other metals and exposed to air. Eliminate oxygen, and serious pitting is possible, often critically weakening the below-deck portions of chainplates. Pair it up with a dissimilar metal, even a different stainless alloy, and both surface staining and pitting can follow.

Corrosion often starts at welds because the heat allows iron to migrate to the surface, iron migrates to the surface, and pitting can start. Shackles and blocks commonly mix 300- and 400-series stainless in search of economy and the best mix of physical properties. Your stanchions may be made of 316 stainless, but it’s a fair bet the mounting bolts are 18-8, which lacks the 2-3 percent molybdenum that gives 316 superior resistance to seawater.

Passivation

Simple exposure to water and oxygen is normally sufficient to build a robust oxide film on 304 and 316 stainless. However, grinding and drilling can often start corrosion, caused by either overheating or the transfer of bits of iron from a contaminated drill or grinding wheel. In these cases, natural oxidation, polishing and even common rust removal acids are not enough to stop the corrosion. To cure this, we need to use a specific process to leach away iron from the surface and from a microscopic layer below.

The industry standards for passivation are ASTM A967 and AMS 2700. These standards describe two general processes, using either 25-45 percent nitric acid or 4-10 percent citric acid. Our technical editor, Drew Frye, has used both processes to passivate chemical processing equipment. Nitric acid is cheaper than citric acid, but given the biting odor and the harm it does  to other metals, we can’t recommend it.

Fortunately, testing has proven citric acid is just as effective, and much safer, requiring only common sense precautions. Diluted to 5 percent for soaking, it is no stronger than lemon juice (6-8% citric acid). Thickened brush-on products are typically 10-20 percent acid solutions and work more quickly. The acid will not only remove surface staining, it will leach iron from the top few layers and encourage the growth of a robust oxide film, protecting the steel far longer than waxes.

Controlling the Process

The main factors influencing passivation are time and temperature. Do not increase the concentration beyond about 15 percent; testing has shown it doesn’t help. Twenty minutes of soaking is sufficient, but a few hours are better. The temperature should be above 70 F, but 50-60 F works if you wait and keep it wet, which is easy in cooler temperatures. NASA settled on 5 percent citric at 180 F for two hours, which we liked for fasteners and fittings. For hardware with polymer parts such as blocks, the temperature should not be more than 120 F.

ASTM lists five specific citric acid procedures. We tested using the three most relevant:

Citric Acids for Passivizing Stainless Steel

Maker	Milliard	Milliard	Nitric Acid	Spotless Stainless	Stellar Solutions	Stellar Solutions	Stellar Solutions	Stellar Solutions	Flitz
Model	Dry crystals	Dry crystals	6% solution	Spotless Stainless	Citrisurg 77+	Citrisurg 2310	Citrisurg 2250	Rust Rescue 200	Premium Pollish
Active Ingredients	Citric acid, 100%	Citric acid 100%, gelled with fumed silica	Nitric acid, 6%	Citric acid, 17% gelled	Citric acid, 10% slight gel	Citric acid, 16% gelled	Citric acid buffers	Calcium sulfate petroleum	Fine compound and wax
Price	8.5	8.5	35	24	26	26	26	28	10
Quantity	32 oz.	32 oz.	16 oz.	22 oz.	22 oz.	22 oz.	22 oz	32 oz.	2 oz.
Cost per Oz. (As Used)	4 cents per ounce	8 cents per ounce	37 cents per ounce	$1.09 per ounce	$1.18 per ounce	$1.18 per ounce	$1.18 per ounce	88 cents per ounce	$5 per ounce

Citric 1: 4-10 percent citric acid, heat to 140-160 F, soak for 4 minutes minimum

Citric 2: 4-10 percent citric acid, heat to 120-140 F, soak for 10 minutes minimum

Citric 3: 4-10 percent citric acid, heat to 70-120 F, soak for 20 minutes minimum. (This procedure is the one recommended by the commercial products we tested.)

AMS 2700 lists these citric acid procedures:

■ 4-10 percent citric acid, 140-160 F, 4 minutes minimum

■ 4-10 percent citric acid, 120-140 F, 10 minutes minimum

■ 4-10 percent citric acid, 100-119 F, 20 minutes minimum

■ 4-10 percent citric acid, 70-100 F, 30 minutes minimum

Some have called chemical passivation a poor man’s electro-polishing. They really aren’t the same thing. Also known as anodic leveling, electro polishing uses electric current to remove 5-10 microns (0.0002 to 0.0004 inches) of material, preferentially from the peaks. This is a micro-polishing method, so the surface must already be smoothed with 400 grit and have no burrs; and the process will only improve surface smoothness about 50 percent.

However, this can make a big difference in corrosion resistance because it deprives crevice corrosion of an easy starting spot (see “Maintaining Stainless Steel”).

What We Tested

We tested Citrisurf 77 Plus, Citrisurf 2310 and Spotless Stainless as commercial products, and nitric acid, citric acid, gelled citric acid, oxalic acid, CLR (lactic acid) and Barnacle Buster (phosphoric acid) as DIY alternatives.

We also tested Flitz Liquid Polish as an example of a top performing metal polish and protectant. You will find these acids in many of the commercial products on the market, such as Davis FSR Fiberglass Stain Remover or Ospho, which found in most hardware stores.

How We Tested

Each of the products was tested on a selection of corroded 316 and 304 stainless fasteners, deck hardware, railings and cutlery. We varied the temperature of the citric acid treatment, from 50 F to 180 F. We tested for effects on brass and aluminum, in case of spills or misuse on chrome-plated parts. We then observed for six months.

Observations

Wear gloves and eye protection. Although citric acid won’t burn you immediately like stronger acids, it will slowly dissolve your skin, can harm your eyes, and will sting like a son of a gun when it finds a cut.

Don’t Let It Dry

Citrisurf 2310 and Spotless Stainless are gelled to keep them in place. To prevent drying you can either add more, wrap it with plastic wrap, or mist lightly with tap water. Generic citric acid solutions can be gelled using Cabosil (see DIY sidebar. No harm occurs when they dry, but they will be less effective.)

400 Series Steel

Spotless Stainless and Citrisurf warn against use on 400 series stainless because it can cause cosmetic discoloration. We tried Spotless Stainless and Citrisurf 77 Plus on a variety of rust-stained kitchen knives. Although 95 percent of the samples came out looking like new, one blade was significantly discolored. The problem is the low 2-3 pH. Citrisurf 2250, formulated for 400 series stainless, is buffered to a pH of 3-4, solving the problem. Always test a small area first.

Heavy Corrosion

A plastic scraper can help knock off heavy corrosion, but anything more aggressive will scratch the stainless and require re-polishing. Brushing with a stiff synthetic brush midway through the treatment removes loosened corrosion and improves results. Only a quick brush is needed, not heavy scrubbing.

Pitting

Don’t expect miracles with severe pitting. If prolonging the treatment doesn’t help and the part is thick enough, try sanding out the rust stains and polishing, followed by a good citric acid soak. The goal of passivation and polishing are to prevent pits; they can’t repair the damage or make them disappear once they become severe.

Cautions

Keep the acid off aluminum, carbon steel and galvanized steel. Although not as aggressive as stronger acids, citric acid will cause some spotting if it stays too long. As with any strong cleaning chemical cover or pre-wet things you would rather not damage.  Keep a hose handy.

Post treatment

Rinse liberally. The thickening agent leaves a white powdery residue you may have to wipe off.

Other Acids and Safety

In addition to safety hazards, acids are also corrosive to metals, and there are obvious risks when cleaning stainless parts. It can drip or splash onto an aluminum toe rail or spar, it can cause corrosion under chrome plating (the base metal is typically brass, bronze, or Zemak, which is largely aluminum), and harm dissimilar components within a mechanism.

Testers put samples of aluminum and brass in properly diluted Barnacle Buster (phosphoric acid), nitric acid, CLR (lactic acid), and citric acid for one hour at 70 F. Not surprisingly, nitric and phosphoric acid are tough on aluminum. In “Descaling Solutions for Boats” we learned that CLR (lactic acid) was the best commercial product for removing scale from aluminum without damaging the aluminum. Nitric acid was also hard on brass. Both lactic acid and citric acid were safe on brass and aluminum.

Although we did not specifically investigate this test, our observations when cleaning some brass alloys on the boat suggest citric acid products are probably also superior for cleaning corroded brass, bronze and copper as well. More testing is needed.

Phosphoric acid actually removes rust faster than citric acid. If corrosion is very heavy, try Barnacle Buster or other rust removal agents first, but then passivate with citric acid for lasting protection. Oxalic acid was moderately effective and lactic acid was ineffective.

Strong acids are damaging to nylon. Keep all rust removers away from nylon sails and rodes. Polyester and Dyneema sheets and halyards are not affected by acid, so you can safely treat snap shackles without removing them.

Finally, don’t forget that old stainless parts that have been subjected to high sustained or shock loads, including standing rigging, mainsheet tackle parts, lifelines and steering gear, may also have work-hardened or fatigued. The former makes them more brittle and the latter may have started micro-cracks. Both can invite corrosion, and neither can be repaired by passivation.

Bottom Line

Citric acid is safe for plated parts and mechanisms that contain bronze or aluminum parts. Staining of aluminum is possible, though no damage occurred during testing.

Spotless Stainless

About 2 to 3 times stronger than lemon juice, Spotless Stainless will sting if you get it in your eyes or on a cut, but it’s relatively safe to work with. It is gelled so that you can brush a thick coating on severely corroded areas. Because the citric acid is more concentrated, you can re-moisten it with a plain water mist as it dries and still maintain effectiveness.

Bottom line: This is our Best Choice for stain removal and lasting protection.

Citrisurf 77 Plus

Citric acid formulated with just enough gel agent to make it cling and wet out. It contains slightly less citric acid that Spotless Stainless, but we cannot document a difference in effectiveness, and industry testing suggests that more than 10 percent citric acid is unnecessary. This product is recommended and sold by Suncor, a major supplier of stainless steel boat parts.

Bottom Line: A Recommended product.

Citrisurf 2310

This is basically the same product as Citrisurf 77 Plus with more gelling agent added, making it better suited to painting and spraying on hardware that is still mounted on the boat.

Bottom line: This is another Best Choice for easy stain removal and lasting protection.

Citrisurf 2250

This product is formulated at a slightly higher pH for use on 400 series stainless. We couldn’t tell much difference, but Citrisurf 77 seemed a little faster and more effective. This is technically the better choice for stainless cutlery, for example, which may be discolored by Citrisurf 77, Spotless Stainless, or stronger citric acid solutions.

Bottom Line: In the field of citric acids, this one did not stand out. Citrisurf 2310 is a better choice for 316 stainless, but Citrisurf 2250 is a good choice for galley knives.

Stellar Solutions Rust Rescue

Rust Rescue is recommended where pitting may be present and additional corrosion protection is needed. Wipe on and let dry. We used it on a heavily pitted stove burner liner.

Bottom line: Without a control, it is hard to say how effective this will be, but the active ingredient, calcium sulfonate, is known to be effective.

DIY Formula Using Citric Acid

This is the active ingredient in citric acid based stainless cleaners. Follow the concentrations recommended by ASTM and NASA—a 4-10 percent solution for soaking in hot water. Increase the concentration to 10-20 percent for spray or brush-on applications to allow for locally heavy corrosion.

However, if you spray this solution on polished stainless, it simply beads up and runs off. To keep in place, you must either add a thickening agent or wrap the corroded areas with rag strips soaked in the solution (see “The Acid Test: Why Buy Something You Can Make Yourself”).

Bottom line: Our Budget Buy, and the leftovers can be used in the kitchen.

Nitric Acid

This is nasty stuff to work with, worse than hydrochloric or sulfuric acid with a biting odor. It is also aggressive to most metals and can damage chrome-plated items. We don’t recommend it for DIY use, but we felt sure someone would ask, since it was the old standard for passivation. We tested both at the ASTM recommended 20 percent, and at 6 percent, since concentrated nitric acid is less widely available and dilution is hazardous. We found it to be no more effective than citric acid.

Bottom line: Don’t use it.

Oxalic Acid

Commonly found in marine hull and metal cleaners, this stuff is hard on the skin and toxic if absorbed. It does a reasonable job of removing surface staining, but it does little to passivate the alloy, and stains soon return.

Bottom line: There are better choices.

Phosphoric Acid

This is another acid commonly used in marine hull and metal cleaners. Our sample product was Barnacle Buster (see “Barnacle Remover Test”). It removes the rust but does not passivate as well as citric acid. It is aggressive towards aluminum and can damage chrome-plated items.

Bottom line: Save it for removing scale and barnacles.

Wichard Wichinox

We did not test this for this particular report, but we have used this on a variety of polishing jobs both on the boat and in the house. It contains phosphoric acid and surfactants, and will polish the stainless, but is not a true passivating agent.

Bottom line: Like many metal polishes on the market, this is just a rust remover, and doesn’t combat the original cause of corrosion.

Lactic Acid

Widely used in a variety of rust-stain removers, lactic acid is the active ingredient in CLR. Although we like CLR for removing scale buildup in plumbing, it is not very effective for this purpose.

Bottom line: There are better choices.

Flitz Liquid Polish

 

This polish is the anchor for a variety of Flitz-branded polishing products and tools aimed at auto and boat enthusiasts who seem to enjoy rubbing and buffing their pride and joy. It is representative of a universe of similar products, each of which has legions of followers (if you can believe all the online chatter). The first thing we noticed is that it was more work.

On one hand, the citric acid promise of “zero work” is only true if the stainless is otherwise clean; if there is adhesive, grease, or just plain gook, you will need to do some buffing anyway. But buffing all the nooks and crannies is tedious business.

Flitz will appeal to those who like to see progress as they work, since there is no waiting. But it also did not protect for as long; once the wax weathered away, it was just a matter of time before staining returned.

Bottom line: Too much work for big jobs.

Conclusions

We were thrilled when citric acid passivation came to the industry, replacing nitric acid and all of the accompanying nuisances. The difference in practicality for boaters is even greater. We’ve never had much interest in polishing stainless or removing stains with oxalic or phosphoric acid. They just come back. But passivation with citric acid offers real hope of protection that is more than cosmetic and produces results that last.

This article was first published on 25 June 2020 and has been updated.