Solutions and Soil
G. HEIM AND W. SCHWENK
4.2 Determining the Corrosion Likelihood of Uncoated Metals
It is a consequence of the action of different pH values in the aeration cell that these cells do not arise in well-buffered media [4] and in fast-flowing waters [5-7].
The enforced uniform corrosion leads to the formation of homogeneous surface films in solutions containing O2 [7-9]. This process is encouraged by film-forming inhibitors (HCOg, phosphate, silicate, Ca2+ and A13+) and disrupted by peptizing anions (Cl~, SC>4~) [10]. In pure salt water, no protective films are formed. In this case the corrosion rate is determined by oxygen diffusion [6,7,10]
where Kw is the diffusion path in the medium and represents a porous film of cor- rosion product. Kw is about 0.2 mm in fast-flowing solutions and about 1.5 mm in free convection [10]. Such diffusion rates correspond to protection current densi- ties Js of O.I to 1 A rrr2, which have also been measured on moving ships (see Section 17.1.3).
In soils the constituents restrict diffusion so that Kw in general rises to over 5 mm. The removal rate is mostly below 30 /am a"1 [11-13]. The danger of corro- sion in soil is generally local corrosion through cell formation or by anodic influ- ence (see Fig. 2-5) and can lead to removal rates of from a few tenths of a millimeter to several millimeters/year.
Fig. 4-3 Schematic representation of the partial current densities in corrosion in free corrosion (a-c) and with cell formation with foreign cathodic structures (d).
sion cells [14] with still essentially uniform material loss. Figure 4-3c, on the other hand, describes free corrosion with extended corrosion cells that can be several kilometers apart. Such situations occur usually with soils that vary locally. The free corrosion potential depends on position—there is no single rest potential (see Fig. 2-6).
If one regards the anodic region in Fig. 4-3c as isolated, the condition for free corrosion (i.e., the equalized current balance) is no longer fulfilled for that section.
It is a case of cell formation in different regions of a pipeline in different types of soil [12]. This situation is no different in principle from cell formation with foreign cathodic structures in Fig. 4-3d [2,3,14,16], only in the latter case the cell voltages and cell currents can be considerably larger. Foreign cathodic structures include
grounding installations and steel in concrete [17,18]. Mostly pitting and nonuni- form corrosion are associated with cell formation.
4.2.1 Corrosion in Soils
Estimation of corrosion likelihood results from consideration of the character- istics of the soils and of the installed object, which are tabulated in Table 4-1 for nonalloyed and low-alloy steel products. Rating numbers, Z, are given according to the data on individual characteristics from which a further judgment can be made using the sum of the rating numbers.
The sum^Q reflects the corrosion likelihood of objects without extended cells as in Fig. 4-3b. This value also characterizes the class of soil, depending on which type of pipeline coating is selected [16]. The sum Bl shows the corrosion likelihood of objects with extended cells as in Fig. 4-3c. This indicates that, in the case of ex- tended objects, the class of soil is by itself not sufficiently informative.
Objects with extended concentration cells can be individual lengths of pipe- line and storage tanks if the makeup of the soil over the surface changes. The distance between anodic and cathodic areas can lie between a few centimeters and a few kilometers.
The sum BE can only be obtained with buried objects and provides information on anodic damage through cell formation as in Fig. 4-3d. More detailed consider- ations can provide information on whether preferential anodic or cathodic regions are formed and how active they are [3,14].
From Table 4-1 it can be seen that a very high corrosion rate can be recognized (Ib and Ic) from a few characteristics alone, and also that the action of coke can be seen as a well-aerated foreign cathode. Furthermore, it can clearly be seen that anodic damage can be much reduced (e.g., by laying a pipeline in high-resistance sandy soils, and homogeneous embedding).
A relatively high degree of corrosion arises from microbial reduction of sul- fates in anaerobic soils [20]. Here an anodic partial reaction is stimulated and the formation of electrically conductive iron sulfide deposits also favors the cathodic partial reaction.
Table 4-1 Characteristics of soils and their rating number Z (DIN 50929, Pt 3 and DVGW worksheet GW 9).
No. Characteristic Z
la Cohesivity (dispersible) (10 to 80%) +4 to -4 Ib Peat, marsh and organic carbon -12 Ic Ash, refuse, coal/coke -12 2 Soil resistance (1 to 50 kQ cm) -6 to +4 3 Water -1 to 0 4 p H ( 4 t o 9 ) -3 to +2 5 Buffer capacity KS4^ and KB1Q -10 to +3 6 Sulfide (5 to 10 mg/kg) -6 to 0 7 Neutral salts (0.003 to 0.01 mol/kg) -4 to 0 8 Sulfate extracted in HC1 (0.003 to 0.01 mol/kg) -3 to 0 9 Relative position of object to ground water -2 to 0 10 Soil inhomogeneity (according to No. 2) -4 to 0 lla Heterogeneous impurity -6 to 0 lib Soil homogeneity vertically -2 to 0 12 Potential t/Cu_CuS04 (-0.5 to-0.3 V) -10 to 0
Table 4-2 shows as an example of the relationship between the results of field experiments [11] and the class of soil. In general, the time dependence of the cor- rosion rate can be represented as follows for t > 4a:
The data for the average decrease in metal thickness in 4 years and the linear cor- rosion rate are given in Table 4-2. In addition, extrapolations of the rate for 50 and 100 years are given, which are of interest for the corrosion likelihood of objects buried in earth. It can be seen from the results that film formation occurs in class I soil. In class II soils, the corrosion rate decreases with time only slightly. In class III soils, the decrease with time is still fairly insignificant.
e 4-2 Evaluation of field tests [11] by class of soil (average ± standard deviation). £ON
l class [14,19] I II IIIr of soils investigated 21 30 27
e in thickness AS 94±37 22 soils: 137±52 12 soils: 268±141 or 4 years in Aim 8 soils: 64±36 15 soils: 220±152 o*""^Or removal rate 6±3.3 16±9.0 55±38 £hn after years in jam sr } 8.o
50 in ^m extrapolated 370±189 22 soils: 873±466 12 soils: 2798±1889 g?e in metal thickness 8 soils: 800±450 15 soils: 2750±1750 or 50 years §2?
100 in jjim extrapolated 670±354 22 soils: 1673±916 12 soils: 5548±3789 |e in metal thickness 8 soils: 1536±864 15 soils: 5500±3800 §;r 100 years P
Table 4-3 Data on local corrosion from field experiments (after 12 years [11]
and 6 years [12]).
Maximum penetration rate vv> in am a~t,max ~ l
Type of soil/reference Class of soil Average Scatter Free corrosion [11] I 30 15 to 120
II 80 20 to 140 III 180 80 to 400 Free corrosion [12] I 133
II 250 III 300
Cell formation [12] 400 (sandy soil/clay soil)
S.:S.= 10
The extrapolated values of decrease in thickness for 50 and 100 years in Table 4-2 are relevant in predicting the life of structural components (e.g., buried foundations of roads and steel retaining walls). These structural items lose their functional efficiency if their strength is impaired by too great a loss of thickness.
The size of test specimens in field experiments [11] is only a few square deci- meters. In these specimens only micro cells can develop according to Fig. 4-3b.
However, the maximum penetration rates in Table 4-3 were greater than expected from the average rate of decrease in thickness. Similar results were obtained from cell experiments with sand and clay soils [12]. Cell action only takes place if the surface ratio Sc/Sa is greater than 10 [13, 18]. In sandy soils, the salt content, and therefore the electrical conductivity, plays a relatively important role [21]. The maximum penetration rates in Table 4-3 are applicable when estimating the life of pipelines and storage tanks. These items lose their functional efficiency if they develop leaks due to shallow pitting or pitting corrosion.
The data in Table 4-1 show the considerable influence of the electrical resistiv- ity of soil. This is particularly so in categories 2, 7, and 10. From a profile of the soil resistance along the course of a pipeline with welded connections or with elec- trically conducting thrust couplings, one can readily recognize anodic areas, and
therefore the locations of areas of increased corrosion. They mostly coincide with minima in the resistance between larger regions of higher resistance [22].
The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of cor- rosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be as- sessed from the sum of rating numbers [3, 14].
Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords con- siderable cathodic protection at UH < 0.2 V. Copper materials are also very resis- tant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14.
4.2.2 Corrosion in Aqueous Media
Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corro- sion can occur in tidal regions, due to the intensive cathodic action of rust compo- nents [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, hori- zontal cells have practically no significance.
4.3 Enhancement of Anodic Corrosion by Cell Formation