In addition to all of that lovely salt, seawater is very hard, nearly saturated with calcium. All it needs is something to react with (uric acid in the head) or localized overheating (engine) to create concrete-like incrustations. Sometimes mechanical removal is possible; a favorite cruiser ritual involves hauling out the sanitation hoses and beating them on concrete to remove internal scale build-up. Heat exchangers can be reamed out with a rod, but most engine and plumbing systems are inaccessible without considerable disassembly.
A less labor-intensive alternative is to dissolve it with an acid descaling chemical, which hopefully will attack the calcium salts selectively, leaving the expensive metal bits undamaged. Flushing a cup of white vinegar each week is a popular scale preventative treatment for the head. Its allowed to soak in the plumbing for 30 minutes or so before flushing through, in hope that build-up will be prevented. Or so we hope.
What We Tested
Descaling chemicals range from weak acids to strong acids, and are available both as generics and as pre-formulated solutions. Among generics, we tested white vinegar, oxalic acid, and muriatic acid (AKA hydrochloric acid or HCl). Among pre-formulated solutions we tested CLR, Lime-A-Way, Rydlyme, Barnacle Buster, and Sew Clean.
How We Tested
First we measured the time required to eat the scale deposits off a collection of used joker valves. Testers diluted each solution to manufacturer recommended ratios and immersed a section of joker valve in the solution and observed. The valve sections were moderately agitated every 30 minutes, and the valve was considered clean when all of the deposits came off with agitation or a very light brushing with no detectable resistance. Muriatic acid was used as a control on one section from each joker valve.
We then placed standard ASTM corrosion coupons in a separate container of each solution to measure corrosion over time. The samples were weighed to the nearest milligram before testing began, and then at two hour intervals over the next 8-72 hours. Each intermediate test required wiping the coupons clean, rinsing with distilled water, and drying. Although metals corrosion is a minor issue when cleaning sanitary plumbing-there should be no vulnerable metals in the system-mariners often use these products to clean piping, inboards, and outboards, and in those applications, the potential for harm calls for caution.
Although the manufacturers sometimes suggest use without dilution, we diluted most products 50 percent (see Value Guide) based on the realistic assumption that there would be residual water in the plumbing.
In minutes, it was obvious which chemicals were fast acting and which chemicals were aluminum-safe. The Barnacle Buster, Sew Clean, muriatic acid, and Rydlyme samples immediately began bubbling wherever lime was present and on the aluminum coupons. The other chemicals-CLR, Lime-A-Way, oxalic acid, and vinegar-also caused bubbling on the lime deposits, though less vigorous, and very few bubbles appeared on the aluminum coupons.
Within one to three hours, depending on the product, the lime deposits were either rapidly disappearing, or becoming loose. What constitutes clean will depend to some extent on the application; the deposits will probably be loosened sufficiently to flush out before they are completely dissolved, which can take considerably longer.
Although few bubbles appeared, the steel coupons also corroded. However, the pattern was reversed; after correcting the exposure time for equal cleaning effect, the chemicals that worked the fastest and did the most damage to the aluminum coupons were the least damaging to steel, and the slower chemicals, which were safe for aluminum, were more damaging to steel, although the damage was less than that done to the aluminum coupons by the fast acting chemicals. Additionally, the steel corrosion was uniform and left a smooth surface, different from the pitting attack observed on the aluminum coupons.
The remaining metal coupons-brass and copper-were not materially affected by any solution during the normal cleaning period. A few milligrams were lost from the copper coupons, which plated loosely onto the steel coupon. This was more of a curiosity than a reflection of noteworthy damage.
All of the solutions are acidic and hazardous as used, but after neutralization or dilution, they contain only relatively harmless or biodegradable components. Obviously, the amount used should be limited to what is needed and disposal should be according to local requirements. Head cleaning acid can stay in the holding tank until the next pump-out (assuming the tank is not aluminum, which is a terrible mistake for holding tanks anyway).
Although we did not test stainless steel, it is known that most common stainless steel alloys, including 316, are vulnerable to stress cracking by hydrochloric acid, even when diluted. While minor exposure of low-stress fasteners to diluted hydrochloric acid for limited time periods poses a minimal risk, do not soak critical stainless steel parts in hydrochloric acid or hydrochloric acid-containing cleaners.
Copper and brass are also susceptible to chloride stress cracking. Stainless steel and brass bellows-type hoses are particularly vulnerable to cracking and should be removed during cleaning with hydrochloric acid-containing chemicals.
Limited tests confirmed that the products that are aggressive towards aluminum are also aggressive toward aluminum, zinc, and magnesium anodes. If you intend to descale a system that includes anodes, temporarily replace these with plain plugs.
We divided the field into two groups: fast acting acids with shorter soak times of one to four hours ( muriatic acid, Sew Clean, Barnacle Buster, and Rydlyme); and slower acting acids requiring soak times of two to eight hours (CLR, Lime-away, Vinegar, Oxalic acid).
This solution of 30-percent hydrochloric acid is by far the least expensive option, but it is also nasty stuff to work with. Chemical burns are quite possible, particularly to the eyes, and the fumes are hazardous to both the eyes and pulmonary system. These are not long term health risks, such as exposure to carcinogens or toxic metals-stomach acid is largely muriatic acid-but rather momentary acute hazards.
Familiarize yourself with safe working practices before you begin. When diluting to pour acid into water, not water into acid, which can create splattering, fumes. We suggest that all solutions be made up outdoors into labeled gallon jugs so that the 30-percent acid is not handled inside the boat or in confined spaces. Keep your head upwind while pouring 30 percent acid and wear goggles while mixing.
Many retailers are now stocking only green hydrochloric acid, which is the same acid packaged at 20 percent to reduce user exposure to fumes. Because the fume concentration is reduced 90 percent at this lower concentration, this is a safer choice, although 50 percent more is required to get the right mix.
We diluted the 30 percent muriatic acid to 4 percent (about one pint per gallon of water). The resulting solution is still acidic and smelly, but much safer to handle. At this concentration it reacted very rapidly with the scale, though not as fast as several commercial products. Cleaning was very effective, but aluminum loss rates were nearly ten times higher than even the other fast acting products.
Bottom line: This is the Budget Buy choice when there is no aluminum around.
Based on phosphoric acid, Trac Ecologicals Sew Clean was the fastest and most effective descaler of the group. There is no objectionable odor, it is somewhat less acidic than muriatic acid, making it a bit safer around metals and skin. Although not as damaging to aluminum as muriatic acid, its still not something wed want to leave inside an outboard engine for longer than required.
Bottom Line: Recommended. This is the best choice to remove heavy scale deposits from sanitary systems.
Similar to Sew Clean, this product, also from Trac Ecological, worked slightly faster, and with slightly greater corrosion rates on the metal samples.
Bottom Line: Recommended for barnacles and heavy duty descaling.
This formula is based on muriatic acid but with effective corrosion inhibitors, it attacked the scale nearly as fast as Sew Clean, but with less corrosive attack on aluminum than generic muriatic acid.
Bottom Line: Recommended for heavy duty descaling of systems not containing aluminum.
The slowest of the slow, the best we can say is that its inexpensive, easy to use, and aluminum-safe. Unfortunately, there are downsides. It was the most aggressive towards steel on a mil/cleaning time basis, the scale removal was glacially slow, accomplishing little even with an overnight soaking, and a number of cruisers reported giving up on vinegar as a preventative method after becoming unconvinced it didn't do much.
Bottom Line: Save the white vinegar for your salad.
Being dubbed the fastest acting of the slow group sounds like a dubious distinction, but this familiar product from Jelmar wasnt that slow, and it did the least damage to aluminum parts for a given amount of calcium removal. On the other hand, it was the safest effective product for descaling an outboard motor.
Like all of the slow-acting products, there is no objectionable odor, and though you should keep all descaling chemicals off your skin, it is relatively safe, no more dangerous than many strong cleaners. Internal sacrificial anodes do not seem to interfere significantly, although removing them may is a good idea for longer soaks.
Bottom Line: This is our Best Choice for cleaning aluminum-containing systems.
Similar to CLR, although based on different chemistry, this product from Ecolabe is gentle to metals and just a little bit slower.
Bottom Line: Recommended for aluminum containing systems.
Also known as wood bleach, it is an effective teak cleaner (Spiffy Teak Tips for the Penny Pincher, PS March 2016) and rust remover. However, it was not effective on head scale and was the most aggressive to steel per cleaning cycle.
Bottom line: There are better choices.
The products tested fell into two categories: those that work slowly but were aluminum-safe (a relative term-exposure time should still be controlled), and those that worked more quickly and were steel-safe. Among the slow but aluminum-safe chemicals, we like CLR and Lime-A-Way.
Among the fast and steel-safe chemicals, you have a choice between many effective and relatively user-friendly descalers, and muriatic acid, which is dirt cheap but stinks to high heaven and requires user caution.
Dont put too much stock in the exact rankings; we know from experience that both the rate and most effective formula can depend upon the chemistry of the scale. The amount and type of scale buildup can vary between applications (heads, barnacles, and engine scale), as well as from boat to boat.
The only really serious mistakes are using muriatic acid on aluminum, leaving any solution in the plumbing or cooling passages longer than recommended, or using a higher concentration than is recommended.
If we were forced to pick an overall winner, we would give the nod to Sew Clean for its combination of speed, reasonable corrosion rates, and relative safety. For descaling engine cooling passages, CLR is a safe bet, following the engine manufacturers guidelines.