Cast irons corrosion potentials

Virtually all metallurgies can be attacked by corrosive bacteria. Cases of titanium corrosion are, however, rare. Copper alloys are not immune to bacterial attack however, corrosion morphologies on copper alloys are not well defined. Tubercles on carbon steel and common cast irons sometimes contain sulfate-reducing and acid-producing bacteria. Potentially corrosive anaerobic bacteria are often present beneath... 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

Similarly, graphitically corroded cast iron (see Chap. 17) can assume a potential approximately equivalent to graphite, thus inducing galvanic corrosion of components of steel, uncorroded cast iron, and copper-based alloys. Hence, special precautions must be exercised when dealing with graphitically corroded pump impellers and pump casings (see Cautions in Chap. 17). 

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

Both silicon and aluminium are added to zinc to control the adverse effects of iron. The former forms a ferro-silicon dross (which may be removed during casting). Aluminium forms an intermetallic compound which is less active as a cathode than FeZn,] . Similarly in aluminium and magnesium alloys, manganese is added to control the iron . Thus in aluminium alloys for example, the cathodic activity of, FeAl, is avoided by transformation of FeAlj to (Fe, Mn)Al/. This material is believed to have a corrosion potential close to that of the matrix and is, therefore, unable to produce significant cathodic activity . 

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

A number of internal components, such as valves, valve seats, cylinder walls, pistons, and rings, will be exposed to hydrogen and water vapor. The potential effects are of two primary types (1) decarburization of steels and cast iron and (2) hydrogen embrittlement of aluminum pistons. Water vapor could cause excessive corrosion of exhaust systems, but this could be minimized by use of titanium. 

An important aspect of cathodic protection is the means to monitor the effectiveness and the criteria for protection. Criteria recommended by NACE (RP0169-96) for the CP of steel and cast iron piping are given in Table 4 [24]. Although several criteria are described for CP of steel structures, the most commonly used criterion is that the steel structure to be protected should be maintained at a potential more negative than —0.85 V versus Cu/CuSOr reference electrode. The primary disadvantages of this criterion are no connection of the potential of the steel to the corrosion rate, and a large difference in protective... 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

Structure-to-electrolyte potential measurements are analyzed to determine whether a structure is cathodically protected these measurements are made by the use of cathodic protection criteria. Unfortunately, no one simple criterion has been accepted by all cathodic protection engineers that can be practicably measured in the field under all circumstances. Guidelines for selecting the proper criterion under various circumstances will be provided below. Guidance concerning the criteria of cathodic protection for external corrosion control on underground structures is found in two recommended practices (RPs) published by the National Association of Corrosion Engineers (NACE). These are RP-01-69 and RP-02-85. A summary of the criteria for steel and cast iron structures follows [8]. 

Cast iron is initially anodic to low-alloy steels and not far different in potential from mild steel. As cast iron corrodes, however, especially if graphitic corrosion takes place, exposed graphite on the surface shifts the potential in the noble direction. After some time, therefore, depending on the environment, cast iron may achieve a potential cathodic to both low-alloy steels and mild steel. This behavior is important in designing valves, for example. The trim of valve seats must maintain dimensional accuracy and be free of pits consequently, the trim must always be chosen cathodic to the valve body making up the major internal area of the valve. For this reason, valve bodies of steel are often preferred to cast iron for aqueous media of high electrical conductivity. 

If the buried metal structure involves contact between different metals (such as mild steel, copper, bronze, brass, aluminum, zinc, lead, stainless steel, cast iron, etc.), it is possible that local galvanic cells can form in the contact areas (Figs. 8-9). Each mettd has its own tendency to corrode. An alternative way to express the metal reactivity is to look at the excess of its free energy (standard electrochemical metal potential) and predict the electromotive force emf) between metals in contact, as a general indication for the corrosion process (Table 2). The metal that has a more positive potential is nobler in the galvanic cell, and it is a cathode. The metal with a more negative potential is more active and acts as an anode in the corrosion cell, e.g., it suffers corrosion. 

Corrosion rates differ only sligthly in the various cast iron types. Grey cast iron (GC) is eroded somewhat more than cast iron with lamellar graphite (GGL) and ductile cast iron with graphite spheres (GCG). Table 29 lists the free corrosion potentials and corrosion rates for uniform surface corrosion in the cast iron types acc. to DIN EN 1561 [87], DIN EN 1563 [88] and DIN 1694 [89]. 

Table 29 Free corrosion potential and corrosion rate of cast iron in seawater...

The greater the difference of potential between the two metals, the greater is the magnitude of bimetallic corrosion. Figure 8.4 shows a valve from a condensate pipe. The cast iron valve was incorporated in AISI 304 stainless steel condensate pipe of a copper heat exchanger. The difference of potential between copper, steel and cast iron caused bimetallic corrosion. 

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