This page was last updated on $Date: 2000/03/06 00:01:07 $
"My Triumph will only leave me when it's rust!" or
There are two different modes through which corrosion can cause a
cooling system failure. The first is the oxidation and removal of
enough metal to cause a mechanical failure. This leads to leaks or
other flow problems. The second is extensive scale formation which blocks
coolant pathways. This leads to reduced flow rates and poor heat transfer.
-Corrosion-
The first mode of failure occurs most frequently in radiators and in water pumps. In radiators perforation is not as common as the heavy corrosion that causes it, because the corrosion by-products often plug the hole they create. In water pumps heavy corrosion causes reduced coolant flow rates, leakage around the housing and can, in truly extreme cases, lead to fracture of the pump.
The second mode of failure commonly occurs when a metal salt dissolves in the hot portion of the system and precipitates in the cold part of the system, usually the radiator. Alternatively, some metals in the radiator may form a heavy, insoluble scale as they corrode leading to blocked tubes.
Another important consideration in understanding corrosion in internal combustion engines is heat flow. Metal that is heat-rejecting has a higher corrosion rate than metal that is heat-absorbing or heat-neutral. Heat-rejection is the transfer of heat from metal to coolant, i.e., in the engine block is heat-rejecting. One reason for this accelerated corrosion is coolant boiling at the surface of such surfaces. To quote a favorite phrase of engineers, physicists and biologists, combined boiling, heat transfer and corrosion are "not completely understood", but bench tests clearly show increased corrosion at heat-rejecting metal surfaces.
"Metallurgy 101: Blacksmithing for Beginners" or
There are a number of metals present in automotive cooling systems. The
most common metals are cast iron, mild steel, copper, brass, aluminum,
high-lead and low-lead solder alloys. Generally speaking, the corrosion
of metals is prevented by the formation of a stable film at their surfaces.
This film might be formed by corrosion products, as when aluminum is
exposed to air, or by the adsorption of some other chemical, such as
silicates, onto the surface.
-Metals-
The corrosion potential for metals is the result of several competing factors. The most important two are the electrode potential, a measure of the tendency of a metal to oxidize, and the protective strength and stability of the surface films. Relatively speaking, the most corrosion prone metals in an engine are aluminum and solder.
- Unlike their behavior in ornamental fences or inner wheel arches, cast iron and steel both have relatively low corrosion rates in automobile engines. The simple, but essential, task of reducing ferrous metal corrosion is accomplished by adding inhibitors to the coolant formulation. Additionally, the corrosion products of ferrous metals are readily dissolved in the coolant and moderately stable in solution. Ferrous metals corrosion is not a major problem in the engine of most cars.
- Copper and brass, an alloy of copper and zinc, have higher corrosion rates then iron and steel. For the record, the TR7 has a soldered brass and copper radiator, and I suspect that every Triumph does. The only alternative to the brass and copper radiator that I know of is an aluminum and plastic radiator that was developed in the late 70's and early 80's. Like ferrous metals, the corrosion of brass and copper can be easily controlled through the use of inhibitors.
- Next we come to aluminum, which is near and dear to the hearts of TR7 and TR8 owners. The corrosion of aluminum can be quite a problem. Based on its electrode potential, aluminum is the most corrosion prone metal in an engine. Only magnesium, sodium and potassium have a greater oxidation potential. The reason our precious engines do not turn into lumps of white powder is that aluminum oxides tend to form stable surface films. However aluminum is particularly sensitive to a process, called erosion-corrosion where a rapidly flowing fluid can remove the protective oxide layer. Erosion can be controlled by limiting the surface flow rate of coolant to 3 m/s or less. This is easily achieved everywhere except at the water pump.
Which leads to a brief aside about the most vulnerable aluminum component in many engines, the water pump. Water pumps and their housings are susceptible to corrosion caused by erosion-corrosion and cavitation. I quote from F. Marks and W. Jetten ("Engine Coolant Testing, 2nd Symposium"):
"Cavitation is the process whereby pressure fluctuations cause the formation and subsequent collapsing of vapor cavities, which exert high mechanical forces on metal surfaces. Erosion-corrosion is the process whereby a flowing fluid surface destroys the protective film giving corrosion free play. The results of both processes are very similar, namely severe localized damage. Cavitation and erosion-corrosion are difficult to separate under test conditions."The rate of cavitation is affected by a number of factors. Increasing the fluid density or fluid boiling point tends to increase cavitation while increasing viscosity, compressibility or dissolved gases tends to reduce cavitation. Some of these properties are effected by coolant additives and the effect on cavitation is one of the characteristics of a coolant package that should be considered.
There is one final problem with aluminum. Some aluminum salts, most notably aluminum phosphate, are not highly soluble in water. Depending on the overall coolant hardness, which is the measure of the total concentration of minerals in the coolant, aluminum salts will precipitate out of solution in the cooler parts of the coolant system.
- The last important metal alloy in the typical coolant system is solder. Solder, like aluminum, is highly susceptible to corrosion. There are two common solder alloys. Low-lead solder is made of about 70% Lead (Pb) and 30% Tin (Sn). High-lead solder is 97% Pb, 2.5% Sn and .5% silver (Ag). As a practical matter, even though it has a decent electrode potential, lead is probably the least corrosion resistant metal in the automobile. This is because lead does not form the stable protective oxide film that aluminum does. Since lead salts are the primary corrosion by-products of solder, it follows that high-lead solder corrodes at a faster rate than the low-lead solder. Unfortunately high-lead solder is distinctly cheaper than low-lead solder and prevalent in most modern, post-1960, automobiles.
Usually the corroded solder forms an insoluble scale at the corrosion site. In fact, a solder joint will often oxidize completely, but remain plugged by the corrosion products. Unfortunately, salts are not well known for there mechanical properties so failure can occur when the salt plug is cracked by vibration.
The primary failure of radiators occurs through the corrosion of the solder joints between the radiator tubes and the inlet or outlet manifolds. Depending on the construction technique, this can be a horrendous and messy problem. Some radiators, called "soft-cored", have cross tubes that are completely solder lined and which can solidly pack with scale. It is the removal of this scale that we call "rodding out" a radiator. A more sophisticated mechanic can remove this scale by dipping the radiator in a caustic solution.
As a final note on metals, clad aluminum has been used to control corrosion and prevent metal pitting and perforation in radiators. An alloy of 99% Al/ 1% Zinc (Zn) is coated on an aluminum surface. In this composite structure, corrosion will proceed preferentially along the alloy surface owing to the lower electrode potential of the Al/Zn composite. If the cladding is applied properly, it can be quite effective at preventing pitting and perforation. If not, the cladding can flake off the surface and create new particulate headaches. An underlying assumption of this strategy is that no radiator lives forever. It only lives a bit longer than the cladding on its surface.
"Add Eye of Newt and Tongue of Bat" or
As reconstituted in an engine, the major component of coolant is, of
course, water. Cheap, non-toxic, inflammable and a good heat transfer
fluid it will probably remain the primary component in cooling systems
for a long time.
-Coolants-
The next major component is the base of the concentrated coolant, as purchased at the store. There are three different bases that commonly are used. Ethylene glycol (EG) is the most common base. Less common is propylene glycol (PG), which has been used for years in Switzerland owing to poison laws and is a recent entry in the U.S. market. Methanol is a third alternative that has been available in Great Britain, mostly for historical reasons.
The function of the coolant base is to extend the liquid range of the coolant. In a 50% mixture, the glycols will lower the freezing point to about -45C and raise the boiling point to about 115C. Another function of the base is to raise the viscosity of the coolant mixture. Higher viscosity mixtures will reduce cavitation at the water pump. PG and EG will both raise the coolant viscosity, methanol will not.
Next, a variety of different chemicals are added to coolants to inhibit corrosion. Cleverly called inhibitors, the function of these additives may be to form a stable, protective film on the metal surface or to alter the solution properties of the coolant. Quite frankly, the precise mechanism of protection of some additives is not known - at least not by anyone who is willing to publish their results. Additionally, the additives in most commercial coolants are usually proprietary. Fortunately for this article, a few brave souls are willing to publish specific information about named inhibitors.
Common corrosion inhibitors include: sodium phosphate, sodium nitrate, sodium tolytriazole, sodium molybdate, sodium borate, sodium benzoate and sodium silicate. Notice that these are all sodium salts. Actually, only the right hand group of these salts is the inhibitor, i.e., benzoate or silicate. These salts dissociates in water, that is, they separate into sodium, with a positive charge, and the inhibitor, with a negative charge. The sodium salts are used because of the high solubility of sodium; you will never ever see sodium deposits in your engine. About the only place you can find sodium metal, outside the laboratory, is inside some high performance valves and heat transfer systems.
Different inhibitors protect different metals. From Vukasovich and Sullivan (also "Engine Coolant Testing, 2nd Symposium"):
"The data show aluminum heat-transfer corrosion was best inhibited by silicate and most poorly by phosphate and borate. ... copper was best inhibited by molybdate and most poorly by benzoate; high-lead solder best by molybdate and phosphate and most poorly by nitrate, silicate and benzoate; low-lead solder best by tolytriazole and molybdate and most poorly by nitrate and silicate; mild steel best by molybdate, phosphate and nitrite and most poorly by tolytriazole and benzoate; gray cast iron best by nitrate and most poorly by benzoate, tolyriazole, and borate; and cast aluminum best by silicate and most poorly by phosphate and molybdate." [Yes, there are two slightly different sentences on aluminum.]Reviewing the most common inhibitors we find: - Phosphate is the most ubiquitous and most controversial inhibitor. It is a well known inhibitor of ferrous metal corrosion, hence trisodium phosphate is used to clean of sheet metal. American car manufacturers have specified phosphate in coolants because it is highly effective at preventing cavitation. Europeans specify non-phosphate coolants because phosphates have a propensity to precipitate in hard water. Also, phosphates have a negative effect on the corrosion rate of aluminum. This beneficial effects peak at concentrations of about 3 g/l and decreases at both lower and higher concentrations. Typical concentrations in coolants range from 0 to 8 g/l.
- Nitrate is included in virtually all formulations because of its efficacy in preventing aluminum radiator pitting, with presumably no negative side effects for other metals. A typical concentration is 2 g/l.
- Tolytriazole is similarly included in virtually all formulations owing to its effectiveness in preventing cupreous metal corrosion. A typical concentration is 1 g/l.
- Molybdate is a broadly beneficial additive. It prevents corrosion in many metals and acts synergistically with phosphates and silicates to prevent corrosion in others. Molybdate also seems to prevent cavitation damage; it is usually selected to perform this function in non-phosphate coolants. Typical molybdate concentrations are 2 to 3 g/l.
- Borate is the most commonly used buffer for coolant systems. Off the shelf, American coolants tend to have a pH of 10 or higher, while European coolants tend to have a pH of 7 to 8.5. In service, the pH of American coolants often drops to 8. Unfortunately, borate tends to have a direct and negative effect on aluminum corrosion. In spite of this, the importance of keeping coolants well buffered is great enough to keep borate in coolant formulations. A typical concentration is 4 g/l.
- Benzoate (and Nitrite, which is not mentioned here) are part of the British Standards Institute's [BSI] Corrosion Inhibited Ethanediol Anti-freeze formulation. Benzoate is more common in European coolants than American coolants and is described as a ferrous metals corrosion inhibitor. Vukasovich and Sullivan found it ineffective in protecting cast iron when present in concentrations less than 5% (an unreasonably high concentration). On the other hand, it does seem to offer protection to mild steel and high-lead solder at lower concentrations. A typical concentration is 5 g/l.
- Finally there are silicates, which appear to be ne plus ultra in protection for aluminum. This is wonderful, but life could never be so simple for Triumph owners. The problem is that silicates are not indefinitely stable in solution. While other additives can be used to stabilize silicates somewhat, I believe that the primary limit to the lifespan of coolants is presence of an adequate silicate concentration. 2 g/l is an effective concentration of silicate.
Bringing all of these inhibitors together, a combination of benzoate, molybdate, borate, nitrate, tolytriazole and silicate is a good additive package that doesn't use phosphates. The non-silicate part of the package is fairly effective in preventing aluminum corrosion, and makes a good back-up system in for an aluminum block engine, should the silicates become depleted.
Other additives appear in coolants as well. These agents are typically used to stabilize the inhibitors or the metal salts which are corrosion by-products. This type of additive is called a sequestrants. Another required additive is the colorant.
"We have questions, We want answers!" or
I have tried to imagine a few common questions that you might ask about
coolants if we were trapped in a British pub and your car was leaking
coolant just outside.
-Conclusions-
Gregory T. Fieldson
The editor of this page is Bob Haskell.
[SOL Home Page]
[SOL Technical Info Index]