Oxygen corrosion galvanized steel

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

The passive film that forms on zinc not only reduces the rate of the anodic process (zinc dissolution), but even hinders cathodic reactions of oxygen reduction and hydrogen development. In conditions of passivity, the corrosion potential of galvanized steel is therefore much lower than that of carbon steel. Values typically measured are between -600 and -500 mV SCE compared to values above -200 mV usually found for passive carbon-steel reinforcement. 

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

Experimental measurements indicated that the change in the thickness of the electrolyte affects the mass transport of oxygen, hydration ofdissolved metal ions, and accumulation of corrosion products [1,54—56]. Dubuisson et al. [57] investigated the atmospheric corrosion of galvanized steel in a micrometric electrolytic droplet containing sulfate and chloride. The measurements were performed in an electrochemical microcell through controlled... 

Reinhart (1976) has reported on the use of zinc alloys and zinc wire ropes that were exposed in the Pacific Ocean at depths of 720-2070 m for periods varying from 123 to 1064 days. The zinc alloy composition was 99.9% zinc, 0.9% lead, and 0.1% iron. The wire ropes were galvanized steel cables of various types. The data obtained from the study are given in Tables 3.23 and 3.24. From the data shown in Table 3.23, the corrosion rate of zinc in Pacific Ocean seawater is seen to decrease with the duration of exposure, except for zinc at the 2400 ft depth, at which the corrosion rate increased with increasing time of exposure. Also, the corrosion of zinc was greater at depth than at the surface. In addition, the report indicated that the corrosion of zinc was not uniformly influenced by changes in the concentration of oxygen in seawater between the limits of 0.4-5.75 mL/L. 

Riickert, J. (1979). Influence of pH value, oxygen content and flow velocity of cold drinking water on corrosion behavior and surface layer formation on galvanized steel tubes. Werkst. Korros., 30, 9-34 (in German). 

Galvanic corrosion is an accelerated corrosion of a metal due to formation of a corrosion cell with a metal or non-metallic conductor that exhibits a higher corrosion potential. For example, if a water pipe made of zinc-coated steel (galvanized steel) is connected to a brass fixture and caution is not taken to electrically isolate the two metals, a corrosion cell is established (Figure 7.5). To simplify the situation, we have replaced, in Figure 7.5, the zinc-coated steel by pure zinc and the brass by copper. The cathodic reaction is the reduction of dissolved oxygen, which takes place on both metals. The corrosion cell formed between the zinc and the copper leads to an accelerated corrosion of zinc near the joint. 

Steel corrodes by electrochemical reactions. In the presence of oxygen, at anodic areas ferric ions and at cathodic areas hydroxyl ions are formed. Aluminum generally corrodes more slowly than steel because of a dense, coherent layer of aluminum oxide. However, aluminum corrodes more rapidly than iron under either highly acidic or basic conditions. Also, salt affects the corrosion of aluminum more than it affects the corrosion of iron. Galvanized steel is protected since zinc acts as a sacrificial anode and a barrier preventing water and oxygen from reaching the steel surface. 

Bare or galvanized steel is subject to corrosion when exposed to aggressive fluids. Corrosion is most severe in the splash zone, where readily available oxygen hastens the corrosion process. Submerged steel shoiUd be coated with a material suitable for use in the anticipated exposure. Where there are concerns regarding the corrosion of steel in contact with process streams, cathodic protection should be provided for steel structures considered to be in a corrosive exposure. This type of corrosion control should be incorporated along with suitable coatings. 

Aqueous media corrosion. Natural water is widely distributed and stored in steel pipe, galvanized steel pipe, and steel tanks. Natural waters, so long as they are reasonably free from aggressive ions, such as chloride and acidic species, are noncorrosive and have been handled satisfactorily by mild steel pipes and tanks for many years. The primary impurities in these waters are calcium and magnesium salts. These salts can form a hard carbonate protective scale on the surface of steel exposed to hard water. Chemically pure, distilled water is, in fact, corrosive, and when the concentration of these salts is low, the corrosion of steel must be controlled by reducing the oxygen present in the water by chemical treatment or by cathodic protection. 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

It also follows that if the solution is stirred the rate of arrival of oxygen at the cathode will be increased. This will result in a corresponding increase in the rate of bimetallic corrosion as is shown in Fig. 1.63 for the aluminium-mild steel couple in stirred 1 - On NaCl solution . The increase in galvanic corrosion rate will be in the inverse relation to the slope of the anodic polarisation curve of the more negative metal, provided that the cathodic reaction is not totally diffusion controlled. 

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