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224 CHAPTER 15 1965 Guide And Data Book anodes. Anode and cathode areas may shift from time to time as the corrosion process proceeds, resulting in essentially uniform corrosion. Reactions at the cathode surface most often control the.. rate of corrosion. Depending upon the nature of the electro-1 lyte, the hydrogen generated at the cathode surface may: 1. Accumulate, so as to coat the surface and slow down the , reaction. This process is called cathodic polarisation.: ' _>_} 2. Form bubbles and be swept away from the surface, thus al- T lowing the reaction to proceed. 3. React with oxygen in the electrolyte to form water, or hy droxyl ion. v less likely will the cathode area become polarized. Alkaline solutions, containing a much higher concentration of hydroxyl ions present than hydrogen, promote polarization of the cathode areas, thus reducing the rate of dissolution at the anode. Relative acidity or alkalinity of a solution is defined as pH, with a neutral solution having a pH of 7. Solutions increase in acidity as the pH decreases, and increase in alkalinity as pH increases. The corrosivity of most salt solutions will depend upon whether they tend to be neutral, acid, or alkaline when dis solved in water. As an example, aluminum sulfate is the salt At the anode, the metal ion going into'solution may reacts with a constituent in the electrolyte to form a corrosion product. With iron or steel, the ferrous ion may react with the., of a strong acid (sulfuric acid), and a weak alkali.(aluminum hydroxide). When dissoived in water's solution of aluminum sulfate is acid in nature and tends to act towards iron as would a dilute add. hydroxyl ion in water to form ferrous hydroxide, and then with oxygen to produce ferric hydroxide (rust). These corro sion products may accumulate-on -the anode-surface and slow down the reaction rate. This process is. called anodic polarization. Since alkaline solutions are generally less corrosive to ferrous systems, it is practical, in many closed water systems, to minimize corrosion by the addition of an alkali or alkaline salt, to raise the pH to 9 or higher. ACCELERATING OR INTENSIFYING FACTORS Moisture Differential Solute Concentration As noted previously, a potential difference between anode and cathode areas is necessary for the corrosion reaction to As has been noted previously,,an electrolyte must be pres ent oh a metal surface for corrosion to occur; no corrosion occurs in dry air. In most natural atmospheres, some moisture is usually present as water vapor, its amount being expressed as percent relative humidity. Vernon*-demonstrated that,.in pure air, practically no corrosion of iron occurs at relative humidities up to 99 percent. But, with contaminants present, such as sulfur, dioxide or solid particles of charcoal, corrosion could proceed at relative humidities of 50 percent or above. Pure air is seldom encountered in practice. During rains, ex posed metal surfaces are completely wetted, thus allowing the corrosion reaction to proceed as long as the metal remains wet. The above conditions apply to iron and.unalloyed steeL Many alloys may develop thin corrosion product films or oxide coatings so as to remain essentially unaffected, by moisture. ,> Oxygen- . With electrolytes consisting of water solutions of.salts.or acids, the presence of oxygen in the media tends to accelerate ,the corrosion rate of ferrous metals by depolarising -the cath odic areas through reaction with hydrogen generated at the cathode, and. thus allowing .the anodic reaction to proceed. In many ferrous systems employed for handling water, such as boiler systems or hot. water heating systems, oxygen;is removed to reduce the. corrosion ,rate: Examples are the proceed. Such a potential difference can be established at dif ferent locations on a metal surface due to differences in con centration of a solute in the media at these locations. Corro sion caused by such circumstances is commonly called con centration cell corrosion. Such cells may be of two general types, metal ion concentration cells and oxygen concentra tion oells. In the metal ion cell, the metal surface in contact with the higher concentration of dissolved metal ion .becomes the cathodic area, and the surface in contact with the lower con centration the anode. The metal ions involved may be a constituent of the media, or may result'from the corroding surface itself. The differences in concentration in the media may be caused by flow of the media sweeping away the dis solved metal ions at one location and not at another. Such differences could occur at crevices, or due. to deposits of one sort or another. The anodic area is to be found outside the crevice or deposit, where the metal ion concentration is least. In the oxygen concentration cell, the surface area in con tact with the media of higher oxygen concentration becomes the cathodic area, and .the surface in contact with the media of lower oxygen concentration becomes the anode. Again, the existence of crevices or foreign deposits on the metal sur face may produce conditions favorable to corrosion through this mechanism. The anodic area, where corrosion proceeds, will be in the crevice or under the deposit. addition of oxygen scavenging chemicals, such.as sulfites, to the system, or. the use of deaeration, equipment to expel the dissolved;oxygen. Presence of oxygen in the media does not affect all alloys in the manner described above. With alloys which develop protective oxide-films,-such as stainless, steels, oxygen may reduce corrosion by. maintaining the oxide film. The media; Galvanic or Dissimilar Metal Corrosion Another factor, which accelerates the-corrosion process may be differences in potential of dissimilar metals coupled together and immersed in an electrolyte. Many factors con trol the severity of corrosion resulting from such dissimilar metal coupling: free of oxygen, might be capable of causing some corrosion of the alloy, but with oxygen present, the oxide film becomes reinforced and thus precludes corrosion.. Solutes. With ferrous materials such as iron and steel, roinenfl adds tend to accelerate the corrosion rate/ whereas alkalies tend to reduce it. Since-the-corrosion reaction at the cathode is related to the relative concentration of hydrogen ions present, the higher the concentration (the more acid the media) 1. The relative differences in position (potential) in the gal vanic series, with reference to a standard electrode. The greater the difference; the greater is the driving forceof the reaction. The galvanic series for metals in sea water is shown in Table 1. - 2. The relative area relationship between anode and cathode areas. Since the amount of current flow, and therefore, total metal loss, is determined by toe potential difference and resists ance of the circuits, a small anodic area will corrode more rapidly, that is, it will be penetrated at a greater rate than a large anodic area. 3. Polarization of either the cathodic or anodic area may reduce ^the potential difference, and thus reduce toe rate of attack of the anode. Corrctfion Qnd- Deposi ; 225 In, general, .it b:not wise to couple a small-exposed area of a less noble-metal with a large area of a more noble metal, in mediawhere the less noble material-may, tend to corrode by.itself- If such couples cannot be avoided, but one of.the dissimilar.metals can be painted or coated with a nonmetafiic coating,1 the cathodic material should be so coated, rather than the anodic one. If the two -materials can be insulated from each.other,Tor example, by the use of an insulated joint in piping systems, the galvanic couple can be avoided. Where this is oot possible, a vaster heavy wall-nipple section of the less noble[material can be employed and can be-readily,re placed when it fails. - . ( In piping systems handling natural waters, galvanic' corro sion is not likely to extend more than 3 to 5 pipe-diameters down toe ID. of the-less noble pipe material. In metal components exposed to the atmosphere, galvanic effects .are likely to be confined to the area immediately adja cent to .the joint. -i . , Stray Current Corrosion . Stray current corrosion is in essence merely one form of galvanic corrosion, the electrical potential driving'the com> don reaction coming from stray electrical currents from an electric generator. Such a phenomenon ran occur on buried or submerged metallic structures. The soil or submergence media provide the electrolyte. The anode, or structure suf fering accelerated corrosion, may be located some distance from the' cathode structure. The most common source of stray currents are direct current electrical systems- such as street railways, welding generators, and electroplating shop gen erators where electrical leaks to ground are difficult to prevent by insulation. The corrosion attack by stray currents is usually restricted to the structure surface in contact with the soil or other sub mergence media. More detailed information on this mechanism is given- in Reference .6. .; Table] 1 .... Galvanic Series of Metals and Alloys ^ -r Corroded End (anodic, or least noble)- . .Magnesium Magnesium alloys Zinc Aluminum 2S . Cadmium Aluminum 17ST ` Steel or Iron Cast Iron Chromium-iron (active) Ni-Rcsist .... - 18-8 Chromiura-nickel-iroh (active) 18-8-3 Chromium-nickel-molybdenum-iron-(active) Lead-tin solders Lead Tin , * .... ., " Nickel (active) . , Inconel (active) . Hastelloy C (active) 1' _. * 1` f ' Copper Bronzes Copper-nickel alloys Monel Silver solder - , .,. Nickel (passive) Inconel (passive) ( i* - - -- Chromium-iron. (passive) 18-8 Chromium-mckei-iron (passive) . . - ' 18-8-3 Chromium-nickel-molybdenum-iron (passive) Hastelloy C (passive) >' Silver ; Graphite Gold ' Platinum Protected End (cathodic, or most nible)' Effects of Stress The presence of stresses in metallic structures rarely has significant effects on the uniform corrosion resistance of metals and alloys. There have been a few instances of stress accelerat ing corrosion with some materials, the more highly stressed areas usually being anodic to the less severely stressed areas: But such mechanisms are more commonly a laboratory curi osity than a practical reality. '. On the other hand, stresses in specific metals and alloys can cause corrosion cracking when exposed to specific corro sive environments. The cracking is often catastrophic in its effects on the usefulness of the particular metal. Almost all metals and alloys exhibit - susceptibility - to stress-corrosion cracking in one or more specific environments; The more common examples are steels in hot caustic solu tions, high sine content brasses' in ammonia, and stainless steels in hot chlorides. For more details regarding specific materials, consult metal producers. ' Stress-corrosion cracking can frequently be prevented by using the susceptible alloy in the annealed or stress-relieved condition, or by selection of-a material known to be resistant to such attack-by the specific madia. * " Temperature There is a common misconception among some engineers that corrosion rates' 'double for every 18 F deg rise in tempera ture. This idea comes from studies of chemical reaction rates where this ratio is a valid approximation. Such a ratio cannot necessarily be applied to corrosion reactions, and generalities as to the effect of temperature cannot be made. ' In systems where the presence of oxygen in the' media promotes corrosion, increase in ' temperature may increase corrosion rate up to a point. Oxygen solubility will decrease as temperature increases, and may approach zero at boiling^ in an open system. Therefore, the.corrosion rate may'de crease, beyond a critical temperature level, due to a decrease in oxygen solubility. However, in a dosed system, from which the oxygen cannot escape, the corrosion rate may continue to.increase with temperature rise- . - For those alloys which depend upon oxygen in the media for maintaining s protective oxide film,' an increase in tem perature and the. corresponding reduction in oxygen content may greatly accelerate corrosion rate by preventing oxide film formation. .. - . ;1 Temperature may affect corrosion behavior in yet another manner, by causing a dissolved salt in the media to precipitate on the surface as a scale, in which case the scale may tend to