Engineer's Guide to Corrosion-Causes, Protection and Control Eight (8) Continuing Education Hours Course #ME1112 Approved Continuing Education for Licensed Professional Engineers EZ-pdh.com Ezekiel Enterprises, LLC 301 Mission Dr. Unit 571 New Smyrna Beach, FL 32170 800-433-1487 [email protected] Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC Course Description: The Engineer's Guide to Corrosion-Causes, Protection and Control course satisfies eight (8) hours of professional development. The course is designed as a distance learning course that overviews corrosion in metals from its causes and effects into protection and control. Objectives: The primary objective of this course is to enable the student to understand corrosion on a variety of metals and methods to mitigate and control corrosion over the life of a material. Grading: Students must achieve a minimum score of 70% on the online quiz to pass this course. The quiz may be taken as many times as necessary to successful pass and complete the course. A copy of the quiz questions are attached to last pages of this document. ii Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC Table of Contents Engineer's Guide to Corrosion-Causes, Protection and Control Environments and Their Affects on Corrosion Processes .......... 1 Forms of Corrosion .............................................................. 13 Corrosion Characteristics and Properties of Metals ............... 61 Corrosion Protection and Control Methods ......................... 117 Corrosion Monitoring and Inspection Technologies ............. 141 Electrochemistry, Kinetics, and Thermodynamics of Typical Corrosion Processes ........................................................... 150 Quiz Questions ................................................... 155 iii Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC 1.0 Environments and Their Affects on Corrosion Processes The environment plays a major role in the selection of materials for corrosion resistance. Environmental factors that may influence corrosion include the environment’s composition, pH level, humidity, wind or water currents, and temperature. These factors exist in the following types of environments, atmospheric, fresh water, saltwater, and soil, which will be discussed in this section. Additional micro-environments, such as specific acid susceptibilities of metals and microorganisms will be covered. 1.1 Atmospheric Environments Atmospheric corrosion can vary widely depending on contaminants present, humidity and rainfall, wind and temperature. Extensive atmospheric testing programs have been conducted to compute corrosion rates of metals and to characterize metals’ susceptibilities to various forms of corrosion. A typical atmospheric testing rack is shown in Figure 1. Such studies have lead to the broad categorization of environments into rural, urban, industrial, marine, and combinations of them. A general characterization of the four main types is listed in Table 1. Deviations of corrosion rates within the four categories have lead to a further subcategorization based upon weather and climate. These additional factors influencing corrosion are temperature, humidity and rainfall. The relative corrosion rates for these environments are found in Table 1. Figure 1 Atmospheric Corrosion Test Rack2 1 Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC Table 1 Types of Atmospheric Environments6 Atmosphere Type Description generally the least corrosive Rural does not contain any significant amounts of pollutants principal corrodants are oxygen and moisture content Urban similar to rural but with sulfur oxides (SOX) and nitrous oxides (NOx) from vehicle and domestic fuel emissions. pollutants of sulfur dioxide, chlorides, phosphates, and nitrates exist from heavy industrial processing facilities Industrial special cases include contaminants of hydrogen sulfide, hydrogen chloride, and chlorine which are highly corrosive to most metals. generally high corrosivity characterized by chloride particles Marine deicing salts used in cold weather regions produce an environment similar to marine Table 2 General Corrosion Rates for Different Atmospheric Environments3 Rate of Corrosion Type of Environment High Tropical Industrial Marine Moderate Temperate Suburban Inland Low Arctic Rural 1.1.1 Atmospheric Contaminants The primary sources of atmospheric contaminants come from chlorides in marine locations, and industrial and automobile pollutants. These contaminants deposit onto metal surfaces where they react primarily with oxygen, water, and free electrons producing metal compounds which have varying degrees of solubility; producing varying increased corrosion rates over non-corrosive environments. The presence of chloride salts in the atmosphere significantly increased the corrosion rates of most metals. In the case of ferrous metals, chloride anions combine with ferrous cations to produce iron chloride. Iron chloride is more soluble than the ferrous hydroxide produced in a benign environment, leading to an increased corrosion rate. Other metals such as copper and zinc produce metal chlorides which are less soluble than ferrous chlorides. These metals therefore experience increased corrosion rates, but not to the extent of ferrous metals. It is notable that deicing salts used on roadways in winter months produce corrosivity in those environments similar to marine atmospheric environments. Sulfur dioxide (SO2) and nitrous oxides (NOx) are found in industrial and urban environments from the burning of fossil fuels. Sulfur dioxide deposited on metal surfaces will react with oxygen and free electrons from the metal surface, producing sulfate ions, as expressed in 2 Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC Equation 1. The sulfate ions lead to the formation of metal sulfates, which in turn react with water to complete the corrosion process, Equation 2. − 2− 22 2eOSO ⎯++ ⎯→ SO4 Equation 1 2− −+ 4 2 2OHFeSO ⎯+ ⎯→ FeOOH 4 3 +++ eHSO Equation 2 As seen in Equation 2, sulfate ions are again produced in the case of ferrous metals, producing a self-contained corrosion process once sulfur dioxide is present. This process may not occur as readily with other metals nor are most of the metal sulfates produced as soluble as iron sulfate. The presence of nitrous oxides may also increase corrosion rates of metals in a like manner, although they do not deposit on metals as readily as sulfur dioxide. There are a few additional atmospheric contaminants that are less abundant or may be found in special industrial environments. Hydrogen sulfide is extremely corrosive to most metals. This compound is readily found in oil-refining and petroleum industries. Hydrogen chloride and chlorine gas have been found to produce higher corrosion rates than chloride salt environments. Ammonia, sulfur trioxide, and smoke particles will also increase atmospheric corrosion of most metals. The typical concentrations of these major contaminants are found in Table 3. Table 3 Typical Concentrations of Several Atmospheric Contaminants3 Contaminant Region Season Typical Concentration (µg/m3) Winter 350 Industrial Summer 100 Sulfur Dioxide (SO ) 2 Winter 100 Rural Summer 40 Sulfur Trioxide (SO3) - - Approximately 1% of the SO2 content Industrial Spring 1.5-90 Hydrogen Sulfide (H2S) Urban Spring 0.5-1.7 Rural Spring 0.15-0.45 Industrial - 4.8 Ammonia (NH ) 3 Rural - 2.1 Winter 8.2 Industrial inland Chloride (Cl-, Air Sampled) Summer 2.7 Rural Coastal Annual (Avg.) 5.4 Winter 7.9 Industrial Inland Chloride (Cl-, Rainfall Summer 2.7 Sampled) Winter 57 mg/l Rural Coastal Summer 18 mg/l Winter 250 Industrial Summer 100 Smoke Particles Winter 60 Rural Summer 15 3 Engineer's Guide to Corrosion-Causes, Protection and Control Ezekiel Enterprises, LLC 1.1.2 Humidity and Rainfall Humidity is also a major factor in determining the corrosion rate of metals, as moisture provides the electrolyte, which is required for corrosion reactions to take place. In general, the corrosion rate increases as humidity increases. The critical level of relative humidity in order for serious corrosion to occur in the absence of other electrolytes is usually taken to be 60%.3 This critical level of relative humidity may vary depending on the impurities present in the atmosphere. Rainfall can increase or decrease corrosion processes. In areas where stagnant water may accumulate, a localized corrosion cell will most likely be the result. However, rain may also wash corrosive deposits off from metal surfaces, decreasing corrosivity. 1.1.3 Wind The wind plays a role in the direction and distance atmospheric contaminants are dispersed. The corrosivity of atmospheric environments and therefore general corrosion rates of metals, are related to their distance and proximity to coastal waters and industrial plants. 1.1.4 Temperature Temperature can have a significant effect on the corrosion of metals, with increased rates of corrosion as temperature is increased. Temperature may also affect the form of attack that the corrosion takes on; for example, changing the temperature may change the corrosion mechanism from uniform to pitting. It can also evaporate condensed moisture on metallic surfaces leaving behind corrosive contaminants. High temperatures can produce a form of corrosion where gas becomes the electrolyte as opposed to a liquid medium. 11.5 Atmospheric