TECHNICAL INFORMATION ON BUILDING MATERIALS

FOR USE IN THE DESIGN OF LOW-COST HOUSING

* ' * *

TIBM - 6l

THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

March l4, 1938

STUDY OF UNDERGROUND CORROSION OE NONBITUMINOUS METAL COATINGS

This paoer gives the results of findings on a series of tests Begun

By the Bureau in 1922 on nonBi tuminous coatings of metals for underground

use. A more detailed account of these findings is reported in Research

Paper RP9S2, "Soil Corrosion Studies1 ',"- from which the material in this

paper has Been taken.

The specimens used in the tests were iron and steel pine and sheet,

coated with three metallic, and ten nonmetallic' coatings. The specimens were Buried in different soils throughout the country,- the more frequently

used types of. soils Being described Briefly as follows:

( 13 ) Hanford very fine sandy loam - fair drainage - alkaline - 21.7$ moisture equivalent.?

(24) Merrimac gravelly - sandy loam - good drainage - acid - 13$ moisture

equivalent

.

(28) Montezuma clay adoBe - poor drainage - 2U.6$ moisture equivalent.

(5S) Muck - very poor drainage - very acid - 57*8$ moisture equivalent.

(42) Susquehanna clay - poor drainage - very acid - 34.8$ moisture equivalent . (43) Tidal marsh - very poor drainage - very acid - 55*^$ moisture equivalent.

(45) Unidentified alkali (C asper, Wyo.) poor drainage - alkali - 1^.3$

moisture equivalent.

•'May Be obtained from the Superintendent of Documents, Washington,

D-. C. .(Price 10 cents).

^The moisture equivalent is the percentage of moisture the soil will

retain against a centrifugal force of 1,000 gravity.

031038-CC 1
, Lead-Coated, Pipe ,

Three separate .tests w.ere made with lead-coated pipe: one for ten

years; one for eight years'; and the third for two years. The coatings for

all three tests had an average thickness of from one, to one and one-half

thousandths of an inch. Together with the coated specimens, uncoated pipes were included as a. mat ter ..of comparison. In several of these comparisons

the rates of maximum penetration were greater for the coated specimens.

This can he explained hy the fact that when the iron or steel beneath the

coating has been exposed, galvanic action between the two metals frequently

accelerates the corrosion of the ferrous metal, and the lead-coated nipe

may be punctured before an uncoated one of the same uipe-wall thickness.

The following tables give comparative results obtained from these

tests in the seven different soils described above.

RATES OP LOSS

( 0 z / f t^ /yr

)

II Soil " Lead-Coated Steel IT B a re S t eel

ii 10 yrs 8 yrs 2 yrs II 10 yrs 8 .yrs 2 yrs

it 13 " .064 .18

II

It 1.70

24 » .033 .08 - .12 .21 - 28 " .314 1.09 - II 1.75 - - 5S " - - • 87

' II - - 1.85

42 " .092 .44 - It •95 .89 - 43 " .726 • 77 - 11 1.47 . 86 - 45 "

!! .19s 1.27 - II

II •79 2.64 -

RATES OE MAXIMUM PENETRATION

(Thousandths of an inch per year)

Soil

1 ! n Lead-Coated Steel

11n Bare . S t e e 1

11 10 yrs 8 yrs 2 yrs

11 10 yrs 8 yrs 2 yrs

17

11

I! 5.4 6.5

11

11 19.3

24 t! 2.2 3-3 - If 2.1 3-7 - 28 It 7-8 9-1 - 11 15.1 - - 58 it — _ l4.g 11 — — 9.0

42 it 4.6 7.4 — 11 7-1 8.8

43 11 18.8 18.0 — 11 8.7 10.8 - 45 11

11 9.2 10.1 - II

II •6.9 16.6 -

It is believed that these data justify the conclusion that thin lead

coatings in general are not suitable for protecting steel pipe against soil

corrosion.

2
Cal o r i zed Pipe

Calorizing is the commercial name for the cementation of a metal sur- face by means of aluminum. The coating is an alloy of aluminum with the

"base metal. There are two processes of calorizing, one known as the powder or dry process, and the other as the din or wet process. Specimens of

nine calorized "by each process were placed in seven soils in 1924. The

rates of corrosion of the calorized specimens, together with the corresponding data for unprotected steel are given in the following table:

Soil

HATES ’ 0? LOSS 0E WEIGHT

( 0 z /?t^ /yr

)

RATES 0E MAXIMUM PENETRATION

(Thousandths of an inch per year)

" Dry

" Calorized

'Wet

Calorized

Fare

Steel

"

. Dry

" Calorized

Wet

Calorized

Fare

Steel

13

If

It 0.137 0.037 1.24

11

» 3.4 4.4 13.0

24 ti .019 .013 0.12 » 2 .4 3.2 2.1

23 H .437 - 1.75 " 5-3 - 15.1

42 ii .452 .038 1.25 " 7.6 4.2 9.2

43 it .861 .U3 U 1.72 " 3.1 7.4 7.4

U5 n

ii .712 .184 1.23 " 3.411

' 4.4 11.9

An examination of this table shows that specimens calorized by the

dry or powder process showed somewhat higher rates of loss of weight and

somewhat smaller rates of maximum' penetration than those calorized by the

dip process.

Galvanized Fine and Sheet

Galvanized iron prepared by the hot-dip process includes an outer

layer of nearly pure zinc; a middle zone of one or more alloys of zinc and

iron; and finally, the metal to which the zinc was apnlied. ‘The thickness

of the alloy layer denends uncn the methods used in galvanizing, and to some extent on the character of the base metal. It is not practicable to

apply an exact amount of zinc or to apply the zinc as a perfectly uniform

coating by hot-dip process as used commercially. Ecr these reasons, and

because of nonuniform soil conditions,- the determination of the effect of

each factor in the corrosion of galvanized specimens under field conditions

would be very difficult, if net impossible, ever, with a very large number

of specimens. From the available data, it is possible to draw general con- clusions only. A summary of data on specimens with nominal two-ounce

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coatings is given in the following table:

AVERAGE RAVE OF LOSS OE WEIGHT

(:0z/ft 2 /yr)

Material

Weight

of

coating

b years

exposure in

78 soils

8 years

exposure in

19 soils

10 years

exposure in

79 soils

Bessemer steel sheet 0.81 0.091 0.145 0.169

Open-hearth iron sheet. •99 .081 .128 .147

Copper-bearing steel sheet 1.07 .095 .13? .132

Open-hearth iron pipe 2.82 .091 .185 .133

A principle applicable to Protective coatings, in general, is that

if the life of unprotected material is short either because of the thin- ness of the material or the corrosiveness of the soil, a protective coating will probably increase the life of the structure sufficiently to

justify its use. If, however, the life of the structure' is long, the desirability of a coating is more questionable. Thus, the use of galvanized

rather than black corrugated iron culverts might be economical, although

the use of large-diameter galvanized water pipes might not be advisable.

Galvanized coatings for underground use should have a weight of at least

two ounces of zinc ner square foot of nine surface.

In tne following table comparative figures are given for the pit

depths in different metals exuosed over a period of 10 years. The galva- nized pipe had a coating of 2.32 oz. per sq. ft. while the copper-bearing

steel had a coating of 1.08 oz. per so. ft.

C0HBITI0H OR RATES OE MAXIMUM PENETRATION

(Mils per yr)

Soil

Bessemer steel

Ho coating

Galv. iron pipe

2.82 oz/ft2

Copper — bearing steel sheet

1.08 oz/ft 2

13 R 0.8

24 2.1 7u Z 28 15.1 R 2.7

42 7.1 R. 1.2

^3 8-7 A R

45 6.9 R id 5

Z = einc still visible over entire surface of a specimen.

A = Alloy later exposed over at least a part of specimen but no rust.

R - specimen rusty over at least a part of surface.

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A further test to determine the relative rates of corrosion of zinc,

steel, and galvanized steel, exposed to five soils for approximately ten

years indicates for the period of exposure involved, and for the soils

from which comparison can he made, that zinc corroded less rapidly than

steel and that galvanized steel corroded least of all.

MI SCELIANDOUS NQKMETALLIC COATIKGS

The following nonraetallic coatings were exposed for approximately two

years to fourteen soils: synthetic rubber, vitreous enamel, rubber paint,

unidentified paint, cashew-nut oil-asbestos-fiber paint, hard rubber,

loaded hard rubber, synthetic resin varnish, chlorinated rubber paint, china

wood oil-mica plastic. The thin coatings employing a volatile solvent were not as satisfactory as the thicker coatings. The types of failures differed

for different coatings and for different soils. Several of the coatings

were ' experimental in nature and the data are insufficient to indicate

whether or not they would be practicable.

Summary

The following are some of the more important conclusions based on the

data presented:

With the exception of the lead-coated specimens, all of the coatings

treated in this report appear to have reduced the rate of corrosion of

steel during the period of the test.

Hone of the coatings have a perfect record for all specimens removed,

although in some cases of short-time exposures the rusting of the protected

pipe was probably caused by moisture entering through the ends of the. specimens and not through the coating.

Lead is sufficiently corrodible in most soils to result in the perforation of lead coatings of the thickness used in these tests within 10

years. After the lead has been punctured, accelerated corrosion may occur because of differences of potential between lead and steel .

Over a 10-year period, the rates of loss of weight of galvanized

steel were from one-half to one-fifth the rates for bare steel.

Galvanized steel corrodes most rapidly in poorly drained acid soils

and in those high in salts.

Tor long periods of exposure, thick zinc coatings are superior to thin

ones. A coating of 2.8 cures? per square foot of exposed surface prevented

the formation of measurable pits in all. but one soil for a period of 10

years

.

The type of ferrous material to which the zinc is applied does not

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have an appreciable effect on the rate of corrosion of galvanized materials during the first 10 years of exposure.

Of a group of nonmetallic and nonbi luminous coatings, vitreous-ename

and hard-rubber coatings afforded the best protection over a two-year period of exposure.

V