TECHNICAL INEORMATION ON BUILDING MATERIALS

TIBM - 22

FOR USE IN THE DESIGN OF LOW-COST HOUSING

THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

Washington, d. c.

July l6, 1936 .

SUBMERGED CORROSION OF FERROUS METALS

The first paper of this series, entitled "Corrosion of Metals

Used in House Construction" (TIBM No. 1), contains a Brief description of three coinmon t 3p")es of corrosion affecting metals in different

parts of a house; viz-, "atmosnher ic"

,

"submerged", and "underground".

Atmosnheric corrosion has been discussed in TIBM's No. 10 and 17 and

underground corrosion vrill be discussed later.

Some Characteristics of Subrn'-uged Corrosion

T,3^ical locations in which submerged corrosion may occur are range boilers, heating-plant boilers, service pipes carrying water

and water-storage tanks, the latter being probably the simplest case.

In a partially filled tank, there are three zones of corrosion:

(a) Metal exposed to the air above the water

(b) Metal at the air-water boundary

(c) Metal covered by the water

As the water level in the tank changes, these zones usually change

Correspondingly.

Corrosion of the part of the tank occupied by air is similar to

the atmospheric type of corrosion described in TIBM No, 10. In an Open storage tank, oxygen is available in unlimited quantity from the

atmosphere which may vary in severity of corrosion according to where

the house is located (See TIBM No. 10). The water necessary for

corrosion is available, however, only while the walls are wetted. As

soon as the surface dries, corrosion is interrupted until the walls

are again wetted as a result of changes in water level within the tank.

In a closed tank, the oxygen needed for corrosion is limited to that

originally present in the air space, plus that brought in in dissolved

form with the water. Since the air in a closed tank is compressed.

07223 6-Z - 1 -

the concentration of the oxygen is increased and the severity of attack

may also he increased. Another factor tending to increase corrosion in a closed tank is the fact that the walls are wet most of the ‘time.

Conditions for corrosion at the air-water hotmdary are usually

very favorable and the attack in this zone is often quite severe.

Since the attack of the submerged part of the tank proceeds by

the action of oxygen dissolved in the water which can dissolve only

relatively small amounts of oxygen, the rate of corrosion in f'is zone

is generally not nearly so rauid as at the "water line".

The corrosion product that forms on the metal in the three zones differs considerably. In the air space, the rust is similar to that

formed by outdoor corrosion and is often rather adherent. The rust

formed in the submerged zone, especially with soft waters (see below),

is generally non-adherent. At the air-water boundary an intermediate

kind of rust is observed. An adherent rust coating has been found

effective in many cases in retarding the rate of atmospheric corrosion,

but on submerged surfaces such protection is not afforded and corrosion

may proceed continuously.

Effect, of Kind of Water Used

The character of the water in the system has an important influence on the durability of tanks or boilers. Two general types of

water are of special importance (a) scale forming, and (b) non-scale

forming. The "soft" waters belong to the latter class and the "hard"

waters to the former. Another class, (c), is water containing sulphates

or chlorides in amoimts sufficient to cause corrosion, particularly, if

scale-forming substances are not present. The effect of scale-forming

waters is most noticeable in boilers or other installations in which the

water is heated, one effect of the heat being precipitation of the scale

forming constituents, A layer is gradually built up on the metal

surface which tends to decrease the corrosion rate. In the absence of

scale-forming substances, especially if the water contains chlorides

or sulphates, corrosion of tanks and boilers may be very severe, A

prediction of the service behavior of the water may often be made with

considerable assurance on the basis of a suitable chemical analysis.

Scale deposits from the water may sometimes be a disadvantage in

iron or steel pipes since they tend to reduce the water carrying

capacity. Such trouble if experienced will, of course, be worse in

the hot water pipe system than in the cold water pipes. Experience

has shown that less difficulty is encountered from this cause when

brass or copper pipes are used. Many advances have been made in the

treatment of waters at municipal filtration plants to reduce their

corrosive properties or their tendencies to deposit abnormal amounts

of scale in water pipes, such for instance, as treatment with lime

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filtration, settling in reservoirs after adding substances to clarify

the water, and other steps of similar nature.

Snhmeraed Corrosion of Range Bpiler^,or HeaUn^lant Boil^e rs

The submerged type of corrosion is least destructive in the

boiler of a hot-watep heating plant because frequent additions o

^ fresh water are not necessary in such systems. The amc^t of air in

a uroperlv installed and maintained hot-water house-neating system is

limited, and after the oxygen has been consumed by corrosion, further

attack is very slight. On the other hand, in steam heating boilers

^ or range boilers to which fresh water is frequently suuulied, corrosion

can be progressive unless scale formation retards it.

Submerged Water Corrosion Tests Made by the Americ an Society f

^

Testing Materials

The American Society for Testing Materials conducted, from 1920

to 1930 , submerged corrosion tests of ferrous metals. Since the conditions were somewhat anaiagous to those in water storage tanks, the

method and conclusions of this test are briefly summarized here.

Samples of uncoated sheets (16 and 22 gage) representing com- mercially available steels and irons, with and without added copper, were set up in long narrow wood boxes through which water was passed.

At Washington, W. C., tap water was used and at toapolis, Md.

,

Severn River water passed. A water level sufficient to cover the

sheets at all times was maintained, and the rate of flow was such t a

there was comulete change of water every I 6 minutes. A sheet was considered to .have failed when perforated at one or more placeji. The

characteristics of the waters affecting corrosiveness are shown 'in

Table 1 and the results of the test in Table 2.

As shown in Table 2, the 22 and 16 gage sheets lasted much

longer at Annapolis (5S uercent longer life for the 22 gage sheets)

than at Washington, although the water at Annapolis (see Table 1), was more highly contaminted with suluhates and chlorides than the

Washington water. The conditions of corrosion, however, were such

that' metal attack uroceeded at Washington without appreciable forma.-

tion of protective scale on the sheets, whereas the reverse was true

at Annapolis. This factor comuensated in large measure for the more

corrosive nature of the Annapolis water. The test is, therefore, an illustration of the importance, in retarding corrosion, of self-formed

protective coatings of this kind. It will be noted, both at Washington

and Annapolis, that the kind of ferrous metal used, whether made with

or without copper, was of secondary importance to the conditions under

which corrosion took place. This latter condition is believed toehold

generally for submerged corrosion and is in contrast to atmospheric

service, for which steels or irons with added copper are considered more

durable, (see TIBM No. 10).

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llo galvanized sheets were included in the submerged corrosion

tests. However, it is known that galvanizing is hcluful in some part's

of a building where submerged corrosion may take ulacc, such as range

boilers, water service -Dines, etc.

General Conclusions

1, Waters that form adherent protective coatings on submerged

metal surfaces are less corrosive than those which do not build' up

such deuosits.

2, Hot-water heating ulant boilers usually suffer least and

steam heating and range boilers most, from submerged water corrosion,

3 , In the "submerged" corrosion of steels and irons, the coitr- position of the metal is subordinate in inroortance to the corrosion

characteristics of the water,

h, A galvanized coating on iron or steel 'may often be advantageous in submerged corrosion service.

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