BUREAU OF STANDARDS TESTS

10.3 BUREAU OF STANDARDS TESTS

The most extensive series of fi eld tests on various metals and coatings in almost all types of soils were begun in 1910 by K. H. Logan of the National Bureau of Standards (now the National Institute of Standards and Technology). These tests continued until 1955 and now constitute the most important source of informa- tion on soil corrosion available [6] . They showed similarity in corrosion rates in

a given soil for various kinds of iron and steels; confi rmation was obtained

208 CORROSION IN SOILS

many soils are listed in Table 10.1 . In addition, data are listed for two soils rela- tively corrosive to steel and for one that is relatively noncorrosive, showing the large variations in corrosion rate from one soil to another. For San Diego soil, the symbol > means that the thickness of test specimen was completely pene- trated by pitting at the end of the exposure period.

Referring again to Table 10.1 , we see that copper , on average, corrodes at about one - sixth the rate of iron, but, in tidal marsh, for example, the rate is com- paratively higher than in most other soils, being one - half that of iron. The rate for copper is normal in otherwise corrosive San Diego soil. Pitting is not pro- nounced, with the maximum depth reaching less than 0.15 mm (6 mils).

Lead also corrodes less on the average than does steel. In poorly aerated soils or soils high in organic acids, the corrosion rate may be much higher (four to six times) than the average. Pitting in some of these soils penetrated the test specimen thickness, accounting for an average maximum penetration greater than the average given in Table 10.1 . Zinc also pitted in some soils to an extent greater than the specimen thickness. In 5 - year tests carried out in Great Britain, commercially pure aluminum was severely pitted in four soils (0.1 to > 1.6 mm,

4 to > 63 mils), but was virtually unattacked in a fi fth soil [9] . Increase in chromium content of steel decreases observed weight loss in a variety of soils; but, above 6% Cr, depth of pitting increases. In 14 - year tests, 12% Cr and 18% Cr steels were severely pitted. The 18% Cr, 8% Ni, Type 304 stainless steel was not pitted or was only slightly pitted ( < 0.15 mm, < 6 mils), nor was weight

loss appreciable in 10 out of 13 soils. However, in three soils, at least one speci- men was perforated [0.4 – 0.8 mm (16 – 32 mils) thick] by pitting. Type 316 stainless steel did not pit in any of the 15 soils to which the alloy was exposed for 14 years. It is expected, however, that pitting would also occur with this alloy in longer

time tests since pitting occurs in seawater within about 2 1 2 years. Zinc coatings are effective in reducing weight loss and pitting rates of steel exposed to soils. In 10 - year tests in 45 soils, coatings of 0.85 kg/m 2 (2.8 oz/ft 2 ) based on one side of the specimen (0.13 mm or 5 mils thick) protected steel against pitting with the exception of one soil (Merced silt loam, Buttonwillow, California) in which some penetration of the base steel could be measured. In later tests

extending up to 13 years, a coating of 0.95 kg/m 2 (3.1 oz/ft 2 ) effectively reduced (but did not prevent) corrosion, even in cinders in which the zinc coating was destroyed within the fi rst two years.

Cinders constitute one of the most corrosive environments. For 4 - or 5 - year exposures, corrosion rates of steel and zinc in cinders was 5 times, copper 8 times, and lead 20 times higher than the average rates in 13 different soils.

10.3.1 Pitting Characteristics

Because the dimensions of all fi eld test specimens were on the order of inches up to about 1 ft (3 – 30 cm), the reported pitting rates represent minimum rather than maximum values. Actual depth of pits in a given time is found to increase with size of test specimen, probably because cathodic area per pit increases (i.e.,

A U OF

ST AND

T A B L E 10.1. Corrosion of Steels, Copper, Lead, and Zinc in Soils (National Bureau of Standards [6] )

S D TE

Maximum Penetration in mils (1 mil = 0.001 in. = 0.025 mm) for Total Exposure Period

Average corrosion rates in g m −2 −1 d (gmd)

Hearth Iron,

Iron,

Steel,

8 - Year

12 - Year

11 - Year

12 - Year

12 - Year

12 - Year

gmd mils Average of several soils

(12 soils) Tidal marsh, Elizabeth, New Jersey

0.53 <6 0.02 13 0.19 36 Montezuma clay, Adobe, San Diego

19 — — Norwood, Massachusetts

Merrimac gravelly sandy loam,

(13.2 years)

210 CORROSION IN SOILS

anode/cathode area ratio decreases), thus accounting for higher current densities at the pits. In addition to this factor, long - line currents or macrocells, if present, increase pit depth over the values obtained on small specimens where such cells do not operate.

The rate at which pits grow in the soil under a given set of conditions tends to decrease with time and follows a power - law equation P = kt n , where P is the depth of the deepest pit in time t , and k and n are constants. It has been reported [10] that values of n for steels range from about 0.1 for a well - aerated soil, to 0.9 for a poorly aerated soil. The smaller the value of n , the greater the tendency for the pitting rate to fall off with time. As n approaches unity, the pitting rate approaches a constant value, or penetration is proportional to time.

Pits tend to develop more on the bottom side of a pipeline than on the top side. This difference is sometimes suffi ciently great to make it worthwhile rotating

a pipeline 180 ° after a given period of exposure in order to increase pipe life. Pitting on the bottom side results from constant contact with the soil, whereas the top side, because of the pipe settling, tends to become detached, producing an air space between the pipe and the soil.