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C

Al

Cu

Cr

Mn

Mg

Fe

Pb

Ni

Si

Zn

88

0.5

2

9

98

0.3

0.2

1

0.6

99.9

62

3

35.5

60

2

39

0.15

18

2

70

9

0.8

0.5

98

3.3

0.4

94

2.5

4

0.015

96

Table 13. Nominal Alloy Composition of Alloys (Compositional data, accessed May, 2007) Elemental composition in weight percent Alloy

AA 360 AA 6061 C12200 C36000 C37700 Type 302

Steel, ASTM A228

Ductile iron ZA-7

Several assumptions were made to assess the possible corrosion problems associated with the various materials of construction of the appurtenances listed in Table 12. For the purpose of these analyses, it was assumed that the alloys are uncoated or that the coatings have been breached due to mechanical damage or scratching. Two scenarios are considered, namely that the alloys will be exposed to the worst-case situation of direct contact with the soils surrounding the buried tank, or that the alloys will be exposed to atmospheric conditions above the soil under the dome. The range of temperature was assumed to be from below freezing to 120 F. The range of humidity was also assumed to occasionally reach 100 percent. The general approach was to use data from previous studies and our experience with regard to how some of these alloys behave when exposed to underground and atmospheric environments.

Underground Environments – Corrosion Processes in Soils

The severity of the underground environment depends upon the soil type and several parameters including: (a) the presence of water; (b) the concentration of oxygen in the water; (c) the conductivity of the water; (d) the presence of dissimilar metals in buried appurtenances; and (e) the presence of bacteria that participate in the corrosion process, known as microbiologically- influenced corrosion (MIC). Each of these parameters is briefly discussed in the following paragraphs.

Soil Classifications

One classification method of soils is based on their water storage capacity. Sandy soils do not retain water well, but soils with a large proportion of clay have excellent water retention. Table 14 lists 11 types of soils listed in increasing order of water retention capacity. Extrapolated penetration data for buried pipes in some of these soil types is presented in the Corrosion Prediction Calculations section.

Alternative Underground Propane Tank Materials, Phase II—Final Report

24

September 2009 Battelle and Lincoln Composites

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