TECHNICAL INFORMATION ON BUILDING MATFRIALS

TIBM - 29

FOR USE IN THE DESIGN OF LOU^COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

August 7. 1956.

CORROSION OF FERROUS METALS UNDERGROUND

Corrosion of Buried, iron or steel differs in several important

respects from the attack on similar material when exposed to air,

(See TIBM Nos. 10 and 17), or when wholly or partly submerged in water

(See TIBI! No. 22). The main factor governing corrosion is the nresence

of oxygen and water together on the metal surface. Impurities in the

air or the water are also iranortant factors.

In soils, the atmosuhere often differs from ordinary air. In

addition to a difference in oxygen content, there may he gaseous impurities such as carbon dioxide, gases from decaying plants, hydrogen

sulphide, etc,, which affect the corrosion rate. Air in the soil is

nearly always saturated with water vapor, but certain imuurities which

affect the corrosion rate under other conditions probably are not present. The access of soil air to the buried metal may vary considerably

in different soils. In general the amount of oxygen available for

corrosion at a given time or area is much less in soil corrosion than

with atmospheric corrosion. Moreover, the amount of oxygen available

varies on different parts of buried metal. Soil water always contains

varying amounts of dissolved salts such as chlorides, sulphates, phosphates and bicarbonates. These affect the ra.te of corrosion directly

and indirectly by their effect on the acid or alkaline character of the

water. Since the pitting tyue of corrosion is frequently associated

with these general conditions, it might be expected that soil corrosion

of metals would oftentimes result in pitting. This has been found to

be the case in extensive investigations by the National Bureau of Standards and also in inspections by public utility companies of oil or gas

pipe lines that have failed by soil corrosion. An imoortant feature of

soil corrosion is the occasional formation on the surface of a more or less adherent coating of corrosior products which may exert some nrotective effect.

An investigation of soil corrosion was started at the National

Bureau of Standards in 1922 and is still in progress. The ferrous

metals used are chiefly plain and copoer-bearing steels, wrought iron,

08043 6-

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open-hearth iron and cast iron. Recently a few special alloy steels have

"been included. Short lengths of pipe of different wall thickness and diameter were "buried in trenches in such a manner that a few specimens of each

material could "be removed at intervals and examined. The pipe specimens

were "buried in 45 different tyues of soils in different parts of the United

States (See attached map)

.

A later phase of the investigation was devoted to a study of bituminous and other coatings such as are frequently used or recommended for

preserving "buried pipes. Galvanized coatings and a few other metal coatings such as lead were also included. Although some conclusions have "been

reached with reference to the "bituminous coatings, definite statements can

not yet "be made regarding the "behavior of galvanized pipes in different

types of soils.

The National Bureau of Standards soil corrosion investigation has

shown that soils differ radically in their corrosive action on iron or steel. The most corrosive soils are heavy, poorly drained soils which

are either very acid or contain excessive quantities of soluble salts.

A porous well-drained soil is generally only slightly corrosive, whereas

a heavy, close-grained poorly drained soil usually gives trouble. Sandy

soils are least corrosive because of their excellent drainage.

Another general conclusion is that in any given soil all the ferrous

materials are attacked at about the same rate. The pipe material supplied

commercially in the greatest wall thickness might therefore be indicated

in those cases where the greater cost of added wall thickness seems justified. It has been noted that cast iron pipe frequently retains much of

its usefulness even after severe corrosive attack, because the network of

uncorroded constitutents of the cast iron is often reinforced by a heavy

coating formed during corrosion which fills the pits and often plugs

perforations.

In very corrosive soils recourse must be had to protective coatings.

Cement coatings have been found, satisfactory except in alkali soils.

Bituminous coatings should preferably be reinforced or covered with one

of the approved commercially used wrapping materials, in order to minimize

the chance of puncturing the coating when covering the pipe with earth or because of soil movements adjacent to the pipe when in service.

The National Bureau of Standards and others have devised useful

electrical tests of simple nature for ascertaining the relative freedom

of bituminous coatings from holes or porosity, and details of these tests

may be obtained from the Bureau. Details may also be obtained from the

same source as to procedure of test or names of laboratories testing the

corrosive effect of soils on metals, which information may often prove

helpful.

The attached charts show locations of tests on the corrosive effect

of soils on various ferrous pipe materials, as well as the effect of soil

drainage on the corrosion of these materials.

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Maximum

Penetration

in

Mils.

The above illustrates the important effect of soil

drainaqe. In poorly drained soils (designated as

"Merced 5ilt Loam" and *Muck/j the corrosion proceeds at a nearly constant increase. In well

drained soils, represented by the other curves

on the qraph, the corrosion almost ceased after

the first 7 or 8 years because of protective coatings which had formed on the pipe surface or

at the bottom of the pits.