Effects of Composition

7.3.2 Effects of Composition

Composition of an iron or steel within the usual commercial limits of carbon and low - alloy steels has no practical effect on the corrosion rate in natural waters or

METALLURGIC AL FAC TORS

soils (see Table 7.1 , Section 7.2.3 ; Table 10.1 , Section 10.3 ). Only when a steel is alloyed in the proportions of a stainless steel ( > 12% Cr) or a high - silicon iron or

high - nickel iron alloy for which oxygen diffusion no longer controls the rate, is corrosion appreciably reduced. For atmospheric exposures, the situation is changed because the addition of certain elements in small amounts (e.g., 0.1 – 1% Cr, Cu, or Ni) has a marked effect on the protective quality of naturally formed rust fi lms (see Chapter 9 ).

Although carbon content of a steel has no effect on the corrosion rate in fresh waters, a slight increase in rate (maximum 20%) has been observed in sea- water as the carbon content is raised from 0.1 to 0.8% [41] . The cause of this increase is probably related to greater importance of the hydrogen evolution reaction in chloride solution (with complexing of Fe 2+ by Cl − ) supplementary to

oxygen depolarization as the cathodic surface of cementite (Fe 3 C) increases. In acids, the corrosion rate depends on the composition as well as the struc- ture of the steel and increases with both carbon and nitrogen content. The extent of the increase depends largely on prior heat treatment (see Section 7.3.3 ) and is greater for cold - worked steels (Fig. 8.2 , Section 8.1 ). Statistical techniques have been used to investigate the effects of minor alloying elements on the cor-

rosion characteristics of commercial carbon and low - alloy steels in 0.1 N H 2 SO 4 at 30 ° C [42] . For the particular steels studied, corrosion rates were increased with increasing carbon content, especially in the range 0.5 – 0.7% C, and by phosphorus.

Both alloyed sulfur and phosphorus markedly increase the rate of attack in acids. These elements form compounds that apparently have low hydrogen over- potential; in addition, they tend to decrease anodic polarization so that the cor- rosion rate of iron is stimulated by these elements at both anodic and cathodic sites. Rates in deaerated citric acid are given in Table 7.5 [43] . In strong acids, the effects of these elements are still more pronounced [44] (see Fig. 7.13 and Table 7.5 ).

Sulfi de inclusions have been found to act as initiation sites for pitting corro- sion of mild steels in neutral - pH solutions [45, 46] . On the other hand, sulfur content has been found to have no signifi cant effect on corrosion rates in acids of steels containing more than 0.01% Cu [42] .

T A B L E 7.5. Corrosion Rates of Iron Alloys in Deaerated Citric Acid and in 4% NaCl + HCl at 25 ° C [43]

4% NaCl + HCl pH = 1 Pure iron (0.005% C)

0.1 M Citric Acid pH = 2.06

3.0 gmd +0.02% P

4.1 39 +0.10% Cu + 0.03% P

37.6 60.6 +0.08% Cu + 0.02% S

140 IRON AND STEEL

Figure 7.13. Effect of alloyed phosphorus, sulfur, and silicon in iron on corrosion in deaer- ated 0.1 N HCl, annealed specimens, 25 ° C [44] . ( Reproduced with permission. Copyright 1965, The Electrochemical Society .)

Measurements of hydrogen permeation rates through cathodically polarized mild steels containing elongated (FeMn)S inclusions show that H 2 S produced at the metal surface by dissolution of inclusions promotes hydrogen entry into the steels. Permeation rates increase with sulfur content of the steel in the range

0.002 – 0.24% S, but only where the H 2 S supply can be maintained by dissolution of inclusions [47] . Elongated inclusions, most commonly manganese sulfi de inclu- sions, provide sites for initiation of hydrogen - induced cracking (HIC), also called

stepwise cracking (SWC), in steels that are used for high - pressure pipelines trans- porting oil and natural gas [48, 49] . HIC is discussed further in Section 8.4 . Arsenic is present in some steels in small amounts. In quantities up to 0.1%, it increases the corrosion rate in acids (less than sulfur and phosphorus); in large amounts (0.2%), it decreases the rate [44] .

Manganese , in amounts normally present, effectively decreases acid corro- sion of steel containing small amounts of sulfur. Inclusions of MnS have low electrical conductivity compared to FeS; in addition, manganese reduces the solid solubility of sulfur in iron, thereby probably restoring the anodic polarization of iron which is lowered by the presence of sulfur [50] . Silicon only slightly increases the corrosion rate in dilute hydrochloric acid (Fig. 7.13 ).

Copper alloyed with pure iron to the extent of a few tenths of 1% moderately increases the corrosion rate in acids. However, in the presence of phosphorus or sulfur, which are normal components of commercial steels, copper counteracts

METALLURGIC AL FAC TORS

the accelerating effect of these elements. Copper - bearing steels, therefore, usually corrode in nonoxidizing acids at lower rates than do copper - free steels [51] . The data of Table 7.5 for several relatively pure iron alloys show that 0.1% Cu reduces corrosion in 4% NaCl + HCl when 0.03% P or 0.02% S is present in the iron, but not for the phosphorus alloy in citric acid. Addition of 0.25% Cu to a low - alloy steel caused reduction of the corrosion rate from 1.1 to 0.8 mm/y in 0.5% acetic acid – 5% sodium chloride solution saturated with hydrogen sulfi de at 25 ° C [52] . These particular relations apply only to the specifi c compositions and experimental conditions reported; they are not necessarily general. Steels containing a few tenths of 1% of copper are more resistant to the atmosphere, but show no advantage over copper - free steels in natural waters or buried in the soil where oxygen diffusion controls the rate.

An amount up to 5% chromium (0.08% C) was reported to decrease weight losses in seawater at the Panama Canal [53] at the end of one year. A sharp increase in rates was observed between 2 and 4 years; after 16 years, the chro- mium steels lost 22 – 45% more weight than did 0.24% C steel. Depth of pitting was less for the chromium steels after one year, but comparable to pit depth in carbon steel after 16 years. Hence, for long exposures to seawater, low - chromium steels apparently offer no advantage over carbon steel. By comparison, however, low - alloy chromium steels ( < 5% Cr) have improved resistance to corrosion

fatigue in oil - well brines free of hydrogen sulfi de. An amount up to 5% nickel (0.1% C) did not appreciably change weight losses of steels exposed to seawater in tests at the Panama Canal [53] extending up to 16 years. Depth of pits, although less for one year of exposure, was greater for long - exposure times for the nickel steels compared to 0.24% C steel (77% deeper for 5% Ni steel after 8 years). Low - nickel steels ( < 5% Ni) have improved

resistance to corrosion fatigue in oil - well brines containing hydrogen sulfi de [54] . Nickel also decreases the corrosion rate of steels exposed to alkalies, and the effect increases with nickel content [55] .

Unim- portant though low - alloy components may be in determining overall corrosion rates in waters or in soils, composition is nevertheless of considerable importance to the galvanic relations and consequent corrosion of steels coupled to each other. For example, because of increased anodic polarization, a low - nickel, low -

7.3.2.1 Galvanic Effects through Coupling of Different Steels.

chromium steel is cathodic to mild steel in most natural environments. The reason is obvious from relations shown in Fig. 7.14 . Both mild steel and a low - alloy steel, not coupled, corrode at about the same rate, i corr , established by the limiting rate of oxygen reduction. When coupled, however, the potentials of both steels, which are originally different, become equal to φ galv . The corrosion rate of mild steel, estimated from the extension of its anodic polarization curve, is now

increased to i 2 , and that of the low - alloy steel is decreased to i 1 . The precise value of φ galv depends on the relative areas of the two steels. Accordingly, steel bolts and nuts used to couple underground mild - steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low - nickel, low - chromium

142 IRON AND STEEL

Figure 7.14. Polarization diagram showing effect of coupling of a low - alloy steel to mild steel on subsequent corrosion rates.

steel or similar composition that is cathodic to the major area of the structure (small cathode, large anode). Should the reverse polarity occur, serious corrosion damage would be caused quickly either to the bolts or to the critical area of weld metal [56, 57] .

Cast iron is initially anodic to low - alloy steels and not far different in poten- tial from mild steel. As cast iron corrodes, however, especially if graphitic cor- rosion 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.