A system's design and operating characteristics are often determining factors at the onset and continuance of corrosion. An understanding of their roles can help to assure optimum design and improved operating characteristics related to corrosion control in the future.

It has already been demonstrated that the coupling of two dissimilar metals will produce a difference in potential between them, with the more negative becoming the anode. The existence of an anode merely indicates that there is a potential for a corrosion current; it does not determine the rate of corrosion. This is governed by other factors, such as the relative areas of the cathode and anode. The rate of corrosion increases in proportion to the ratio of cathodic area to anodic area.

In the evaluation of a nickel-steel couple, for example, the galvanic series indicates that nickel is cathodic to steel. In a tube-to-tube sheet joint in a seawater heat exchanger, using steel tubes and a nickel tube sheet, extensive corrosion of the steel tubes could be expected. Reversing the situation, results in far less corrosion of the steel because the ratio of cathodic to anodic area is small.

The effect of relative areas also applies to a single metal. A small rupture in the oxide film of a passivated metal will cause extensive corrosion, because a small anode has been created in a large cathodic field.

The relative-area effects become more important as the conductivity of the solution increases. In dilute solutions, only the cathodic area immediately around the anode is important, because the solution's conductivity is low. In brackish, or seawater, relative area effects are extremely important.

TEMPERATURE

The extent of corrosion increases with temperature. For example, in a domestic water system, a rise in temperature from 60°F to 176°F may increase corrosion as much as 400 percent. An increase in inhibitor levels would, consequently, be necessary to minimize this problem.

In general, as temperature rises, diffusion increases and both overvoltage and viscosity decrease. Increased diffusion enables more dissolved oxygen to reach a cathodic surface, thereby depolarizing the corrosion cell. Overvoltage decreases cause depolarization by hydrogen evolution. A decrease in viscosity aids both depolarization mechanisms because it favors solution of atmospheric oxygen and enhances hydrogen evolution.

In an open system, corrosion rates increase with rise in temperature, despite the decrease in the solubility and availability of dissolved oxygen, because diffusion rates have also increased. The net effect is an increase in the amount of dissolved oxygen reaching the metal surface. This trend continues until about 170^eF, when the loss of dissolved oxygen exceeds the amount made available by diffusion, and a decrease in the corrosion rate again occurs as seen in the graph below.

In a closed system, corrosion increases steadily as temperature rises since oxygen under pressure cannot escape. The net effect, therefore, is an increase in diffusion rates.

Any temperature variation within one piece of metal will cause the warmer portions to become anodic to the cooler areas; a condition which explains active corrosion in systems subjected to unequal heat transfer, and a problem often experienced in fouled heat exchangers. This is shown in the figure below.

Some metals or alloys change their electrical potential as temperature rises. The zinc coating on galvanized steel becomes cathodic to the ferrous portion at about 150°F and no longer provides corrosion protection.

HEAT TRANSFER

Heat transfer surfaces such as those in cooling water systems are particularly difficult to protect. This is attributed to high skin temperatures along the metal surface. These can lead to "hot wall effects" in which oxygen is released from solution at the hot metal surface and promotes the formation of a differential aeration cell.

VELOCITY

There are two categories of water flow, laminar and turbulent. Laminar flow is low in velocity and may not be consistent across the metal surface. Turbulent flow is at a rate and distribution nearly that of "plug flow" (maximum velocity turbulent flow). These are shown in the figure below.

Velocity distribution in laminar and turbulent flow, (a) Low velocity laminar flow, (b) High velocity turbulent flow. (c) Very high velocity plug flow.

For inhibited water there is a compensating factor. As oxygen diffusion to the metal surface increases, so does inhibitor diffusion. Therefore, less corrosion inhibitor is required at higher velocities. Conversely, cooling water systems in a standby condition, or low velocity areas in a cooling system (e.g. shell side water flow heat exchangers), are difficult to protect at normal inhibitor levels.

DISSIMILAR METALS

The direct contact to two dissimilar metals in a conductive solution may cause a potential difference between them (the metal of lower potential becoming the anode) and result in formation of an active galvanic cell with subsequent corrosion of the anodic metal. A listing of metal potentials in their descending order (the Galvanic Series), is shown in the figure II-b. Any metal coupled to one above it will become the anode. If a galvanic cell is set up by the coupled metals, hydrogen will be reduced on the cathodic metal and absorbed on its surface.

The extent of corrosion is determined by many of the factors discussed previously; relative areas, solution conductivity and polarization mechanisms being the most important. If the ratio of cathodic to anodic surfaces is low, galvanic corrosion will be limited. Solutions of low conductivity also limit galvanic attack because only the immediately adjacent cathode areas are associated with the anode, negating any relative area effects favorable to corrosion. Any polarizing condition in the system would obviously decrease the potential difference between the metals and neutralize the corrosion reaction.

In the design of equipment where galvanic corrosion may be a problem, dissimilar metals should be separated by a non-conductive substance such as a dielectric union, plastic sleeve or insert.

Galvanic cells may be used to advantage to protect a given metal. In many situations, iron is protected by coupling it to a less noble metal such as magnesium. In such a cell, the iron becomes the protected cathode and the magnesium, the sacrificial anode.

METALLURGY

Metals are never absolutely flat, plane structures. All have surface flaws such as scratches, crevices, etc. In which the potential for dissolution increases. Those areas will become anodic to the rest of the metal. A stressed metal would normally set up anodic sites at certain intergranular boundaries. Anodic site formation may result from a number of causes detectable under microscopic inspection. Inclusion of a non-homogeneous metal or other metallic compound in the grain structure results in the formation of a small galvanic cell in that area. Two adjacent grains of different density might create a corrosion cell. Precipitation at metal grain boundaries will cause a corrosion cell to form, especially if the precipitate is more noble than the metal itself.