Cooling System Corrosion
As a general rule of thumb, the two main types of corrosion within cooling tower systems are general and localized corrosion, but what distinguishes each corrosion process?
Corrosion can be defined as the destruction of a metal by a chemical or electrochemical reaction with its environment. In open recirculating cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency due to loss of heat transfer-the result of heat exchanger fouling caused by the accumulation of corrosion products. For corrosion to occur, a corrosion cell, consisting of an anode, a cathode, and an electrolyte in solution must be present. Metal ions dissolve into the electrolyte solution (water) at the anode and electrically charged particles (electrons) are left behind which flow through the metal to the cathode where electron-consuming reactions take place. The result is the loss of metal and often the formation of a deposit in a process referred to as deposition. Corrosion presents itself in several ways, but the most prevalent form in cooling systems are general corrosion, localized attack, and galvanic corrosion.
General corrosion exists when corrosion occurring in a system is uniformly distributed over metal surfaces. In the case of mild steel, considerable amounts of iron oxide are produced by generalized attack which contributes to fouling problems. The surface effect produced by most direct chemical attacks (as by an acid) is a uniform etching of the metal. While this is the most common form of corrosion, it is generally of little engineering significance, because structures will normally become unsightly and attract maintenance long before they become structurally affected.
Important factors that affect corrosion in cooling water systems are Oxygen and other dissolved gases, dissolved or suspended solids, alkalinity or acidity, the velocity of flow, temperature, and microbiological activity. Dissolved oxygen is essential for the cathodic reaction to take place. Solids that are dissolved in solution increase the electrical conductivity of the water (electrolyte). The higher the dissolved solids, the greater the conductivity. Dissolved sulfates and chlorides are particularly corrosive.
Localized (pitting) attack exists when only small areas of the metal corrode. Pitting is the most serious form of corrosion because the action is concentrated in a small area and causes perforation of the metal in a short period of time. Forms of localized corrosion include pitting, selective leaching (e.g., dezincification), galvanic corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, and microbiologically influenced corrosion (MIC). Another form of corrosion, which cannot be accurately categorized as either, is erosion-corrosion.
Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. In a galvanic cell, there is a potential difference when two metals with different electrical potentials are connected. The metal with the higher electrical potential becomes the anode, and the lower being cathodic.
Figure 1: Galvanic Series (From Cathodic – Top to Anodic – Bottom)
A current will flow from the anode to the cathode. The sacrificial anode dissolves (corrodes) to form ions and as ions drift into water, they either stay in solution or react with other ions in the electrolyte. Galvanic corrosion rates depend on the electrical potential between the two metals, illustrated in Figure 1. Metals near the bottom of the series act as anodes and suffer corrosion when coupled with those nearer to the top. Galvanic couples of metal nearer together in the series will tend to corrode more slowly than those that are further apart. A large anodic area coupled to a small cathodic area produces very little galvanic current. However, a large cathodic area coupled to a small anodic area may result in severe corrosion.
Erosion corrosion does not result from an active corrosion cell, rather the physical loss or removal of metal caused by the presence of high solids and/or particulate material moved by the velocity of water circulating in cooling systems. Suspended solids influence corrosion by erosive or abrasive action, and they can deposit on metal surfaces and form localized corrosion cells. Acidic and alkaline waters can dissolve metal and the protective oxide film on metal surfaces.
Microbiologically Influence Corrosion (MIC)
When bacteria and/or biofilm are determined to be a large contributor to a deposit, under deposit corrosion can be identified as being indirect MIC. Sulfate-reducing bacteria, which causes a form of direct MIC, can cause severe and rapid corrosion. Microbiological growth promotes the formation of corrosion cells by the mechanism of under deposit corrosion or by oxygen depletion. Some microorganisms form corrosive byproducts such as hydrogen sulfide which in water forms sulfuric acid. The high localized concentration of acid then causes rapid pitting corrosion of metal surfaces.
There are several methods used to prevent or minimize the effects of corrosion in cooling systems. Mechanically, using corrosion resistant materials or applying protective coatings are simple and cost-effective approaches. Where corrosion is already present with considerable damage, protective coatings can help prolong the lifespan of equipment. Sacrificial anodes can be introduced to provide cathodic protection. Chemically, the introduction of a pH adjuster or corrosion inhibitors provide ongoing protection. The traditional and widely accepted method of using non-oxidizing biocides in conjunction with a halogen residual will help prevent MIC. There are several inhibitors, but the ultimate choice depends upon factors such as water composition, heat load, metallurgy, and system volume.
A tremendous amount of material has been published relating to corrosion and the repercussions to cooling systems, and for good reason. Unlike scale, corrosion cannot be reversed once damage has occurred. And with the expense associated with replacing or installing these systems, the prevention of corrosion comes with much value. When it comes to being able to treat a cooling tower, thorough understanding of the foundational principles such as corrosion allows water treaters to better understand what makes our jobs so important.