Corrosion's effect must be considered when designing open- or closed-loop hydronic systems.
Designing an open- or closed-loop hydronic system for domestic or commercial applications requires an appreciation for the role that corrosion plays in degrading system components over time. The variations in the factors controlling corrosion account for the complexity of the corrosion problems. With corrosion in mind, the plumbing engineer needs to carefully select the required pumps, valves and piping based on the susceptibility of their metal alloys to the corrosion process. This article will present a brief overview of open- and closed-loop heat transfer systems, and then focus on ways to prevent or minimize the oxidation process that is at the heart of the corrosion problem.
Corrosion in a water transport system is the destruction of materials by chemical or electro-chemical (galvanic) reaction. Most corrosion processes are electro-chemical in nature, like a dry-cell battery. There is a flow of electricity between an area of the metal surface through the solution capable of conducting an electrical current. The eating away of the metal is at the point (anode) where the current enters the solution.
For corrosion to occur, there must be a release of electrons and a formation of metal ions through oxidation of the metal at the anode. There is a cathode in this cell that is accepting electrons, forming negative ions.
Reactions at the cathode surface control the rate of corrosion. Depending on the chemical make up of the solution, hydrogen will generate at the cathode surface to:
- accumulate to coat the surface and slow down the process;
- form bubbles and be washed away by the moving fluid, allowing the corrosion process to proceed;
- react with oxygen in the solution to form water or a hydroxal ion.
At the anode, the metal going into solution may react with a component of the solution forming a corrosion product. In the case of iron or steel pipe, ferrous hydroxide is formed. If oxygen is present, ferric hydroxide (rust) is formed.
The factors affecting corrosion in a system are:
- the amount of oxygen;
- the chemical makeup of the solution;
- galvanic corrosion resulting from the coupling of dissimilar metals;
- flow rates.
These factors will be discussed throughout this article.
The essential difference between a closed-loop (Figure 2) and an open-loop (Figure 1) system is whether there is a requirement for an opening or outlet to an outside water or atmospheric source. A typical closed-loop heat transfer system has all points sealed from the atmosphere; water captured in the system circulates repeatedly, and the system's air separator removes virtually all of the air present in the water. An example would be a residential or commercial heating or cooling system that is pressurized.
If the system malfunctions because of a leak or other failure (like a waterlogged expansion tank), its relief valve dumps water. This requires opening the system and allowing fresh water in. New water brings its air content with it, and more work for the air separator. A closed-loop system, however, functions most routinely with a limited and non-renewable amount of air in the water present in the system. In this regard, over the course of its working life, it encounters little outside water or direct air to invite corrosion.
An open-loop system is essentially the opposite: A point in the system is open to the atmosphere. The typical home is a good example; whether it be from the city main or through a backyard well, oxygen-rich potable water is continually being fed into the system for domestic water use as cold water or heated for hot water use. The hot water is sometimes recirculated by the fixtures to provide instant hot water.
A large air conditioning system would circulate water through the condenser of the refrigeration system, removing heat and transferring heat to a cooling tower typically located on the roof. The water is sprayed into moving air where the process of evaporation releases some of the heat. The process also introduces air into the water. The loss of water because of the evaporation process is made up with new water. The mineral content and dissolved air can accumulate if not properly treated. The solubility of oxygen in water and the effects of pressure and temperature are shown in Figure 3.
The water coming from the building water supply can contain different minerals and dissolved gases, depending on the location of the water source. The gases can make the water corrosive or the minerals can build up on the metal surfaces, inhibiting corrosion. If the buildup is extensive, the heat transfer efficiency of the heat exchangers is reduced.
How oxygen corrodes
Air dissolved in water consists of about 30 percent oxygen (the rest is non-corrosive nitrogen.) Given time, oxygen degrades metals through an electro-chemical process of internal oxidation. What happens is that metal gradually gets converted to an oxide, becoming thinner and weaker in the process. This results in rust, grooving and pitting, hot wall effect and fatigue, which are but a few ill effects of the overall process.
Water possesses several unique properties, one being the ability to dissolve, to some degree, every substance present in the earth's crust and the surrounding atmosphere. Because of this solvent property, water contains certain impurities. These impurities can be a source of trouble by despositing themselves in water lines and other surfaces that come into contact with water.
Galvanic corrosion is when different metals come in contact with each other. When this happens an electric current is generated.
Metals are listed in a series according to their activity. The highest in the series is the corroded end (anodic), least noble. The lowest end of the series is the protected end (cathodic), most noble. The tendency of a metal to give up electrons and go into solution is dependent on there being four essential elements:
- a more reactive metal called the anode;
- a less reactive metal called the cathode;
- a water solution environment called the electrolyte;
- contact between the two metals to facilitate electron flow.
To prevent this type of corrosion:
- Eliminate contact of dissimilar metals by using insulating couplings or joints.
- Remove dissolved oxygen.
- Use protective coatings or oxide coatings that provide a barrier between the corroding metal and its environment.
Corrosion of iron in water
Iron corrodes in water in the absence of oxygen because it is less noble than hydrogen. Iron replaces the hydrogen ion in water. In pure water the reaction product, ferrous hydroxide, elevates the pH by providing hydroxide ions. The elevated pH reduces the amount of H+ ions available to react. The reduction in hydrogen ions tends to retard the corrosion reaction. Secondly, the Fe++ reaction products, Fe(OH)2, concentration is increased, reducing the electric potential or driving force for the reaction to proceed. If the temperature of the water rises, ferrous hydroxide is converted to magnetite in the absence of oxygen, thereby forming a somewhat protective film barrier, further retarding the reaction by physically separating the reactants. Above 120°F only magnetite is found.
The corrosion of iron should be a self-limiting reaction proceeding to equilibrium and then slowing down to an almost immeasurable rate. This slowing down of the corrosion rate is the reason closed-loop heating and cooling systems can be constructed of steel and iron and have a good service life. However, in heat transfer systems, the corrosion problem is worse if oxygen is renewed because the hotter the system, the more rapid the oxidation process. In an open system, the oxygen boils off at 212 °F. (Figure 4)
When oxygen is present, the corrosion mechanism of iron proceeds further. Ferrous hydroxide is unstable in the presence of oxygen, and ferric hydroxide will be formed. While ferrous hydroxide is a corrosion reaction retarding agent, ferric hydroxide is not. The iron-water-oxygen reaction forms ferric oxide. Ferric hydroxide is virtually insoluble in water and precipitates out of solution. The precipitate formed by ferric hydroxide is not protective. (Figure 5)
Oxygen corrosion is easily recognized by the large pits produced. Oxygen pitting begins at weak points in the iron oxide film and continues at the same location. The type of pit developed is influenced by the pH. At low pH, the iron oxide layer is not particularly protective, so pits tend to be larger. As the pH level increases, a protective oxide layer is more effectively maintained, resulting in pits of smaller circumference. (Figure 6)
Active oxygen pits contain reduced black oxide along the concave surface of the pit, while the surrounding area above the pit becomes covered with red ferric oxide. Black iron oxide indicates that the pit is active, whereas the presence of red iron oxide indicates no activity.
Oxygen corrosion of copper
Corrosion of copper by oxygen generally results in formation of cupric oxide, and the reaction is self-limiting. If ammonia is present, the copper oxide film cannot become permanently established. The oxidation of the base metal takes place and is washed away.
High concentrations of carbon dioxide in the water with pH values of less than 8 have an effect similar to ammonia in dissolving th copper oxide film.
Carbon dioxide corrosion
As carbon dioxide dissolves in water, the pH is depressed through the increase in hydrogen ion concentration. Carbonic acid is formed and promotes the iron corrosion reaction by supplying a reactant H+.
Effects of water velocity
As stated earlier, copper builds up a cupric oxide layer on the surface and protects the metal from further attack. If the water velocity is above 4 feet/second, the oxide layer is destroyed or does not form. Without this oxide layer the metal will tend to deteriorate. The chart, "Behavior of Copper Alloy Condenser Tubes Under Velocity Conditions," is a good reference for maximum velocities for different copper alloy materials. (Figure 7)
Other agents also come into play. For example, in an open domestic hot water system, exposed metal tubes are attacked by oxygen. If the water passing through them contains magnesium or calcium carbonates, which can be a corrosion inhibitor, deposits form. However, if these deposits continue to collect, the heat transfer function will become degraded. Hot spots ("hot wall effect") will form that can actually melt away the metal. In steam systems, gases in water, like carbon dioxide, carry over and condense with the condensate and become carbonic acid, which corrodes condensate piping. Water being fed to boilers should pass through a reactor to remove air and carbon dioxide gases.
Cast iron, brass, bronze, stainless steel and copper
As mentioned earlier, the tendency for the wetted surfaces of copper pipe to build up an oxide layer makes copper, in most cases, a universal material for both hot and cold, open and closed systems. In larger copper pipe (above 3 inches), steel is more economical for cold water and closed-loop hot water systems. The exception would be cooling tower piping, but treatment would be required.
Metal alloys have been found to be useful agents in combating the oxidation process, and many hydronic system components can be selected in one or more alloys, depending on the application. For example, cast iron and brass, a mixture of copper and zinc, are best employed in both hot/cold closed-loop systems. Bronze, comprised of copper and tin, stands up well to the rigors of an open-loop hot-water system. Because cold water, with dissolved oxygen, is not as corrosive as when heated, cast iron, bronze fitted pumps can be used.
The corrosion process could be significantly reduced if the oxygen could be consumed before entering the building or city mains.
Brass is especially susceptible to dezincification if the zinc content is greater than 15 percent. If the water is low in hardness and slightly acidic, the zinc will be lost from the brass through corrosion, leaving a porous zinc-free copper with very little mechanical strength. Some brass alloys using arsenic and/or tin have been found to be less susceptible to dezincification For hot water open systems, the use of all-bronze pumps is an economical solution. The 300 series stainless steel, which is a steel alloy containing chrome, nickel and manganese is corrosion resistant in closed and open systems. Material and fabrication costs along with shape limitations are factors in selecting stainless steel.
Beyond selecting metal alloy system components that resist oxidation, there are various treatments that can slow the corrosion process. These include coatings on wetted elements of pipes to create an oxide barrier, placement of a corrosion-resistant glass glaze or cement lining inside water heater tanks, and use of a "sacrificial element" such as a zinc rod. Some water treatments are designed to accelerate an oxide layer buildup to protect the metal surfaces from corrosion.
Because of the differences of water chemistry from one location to the next, there is no one treatment that will satisfy all conditions.
The services of a water treatment company should be used to test and treat the water for a particular application.
Most small closed-loop, low pressure and temperature systems (250 °F) would not need any treatment. The open cooling tower has another set of factors such as bacteria reactions, mineral build-up, oxygen content, organic growth, algae, fungi and bacteria. All these conditions can be handled by the expertise of a water treatment company.