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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 OF

The first paper of this series, entitled "Corrosion of Metals Used in House Construction" (TIBM No. 1), contains a Brief descrip- tion 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 and water-storage tanks, the latter being probably the simplest case. In a partially filled tank, there are three zones of corrosion:

(a) 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, 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.

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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 is similar to that formed by outdoor corrosion and is often rather adherent. The rust formed in the submerged zone, especially with soft (see below), is generally non-adherent. At the air-water boundary an intermediate kind of rust is observed. An adherent rust 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 influ- ence 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 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 or pipes since they tend to reduce the water carrying capacity. Such trouble if experienced will, of course, be worse in the hot water system than in the cold water pipes. Experience has shown that less difficulty is encountered from this cause when or 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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after adding substances to clarify filtration, settling in reservoirs similar nature. the water, and other steps of HeaUn^lant Boil^e rs Snhmeraed Corrosion of Range Bpiler^,or

least destructive in the The submerged type of corrosion is plant because frequent additions o boiler of a hot-watep heating ^ systems. The amc^t of air in fresh water are not necessary in such hot-water house-neating system is a uroperlv installed and maintained corrosion, further and after the oxygen has been consumed by limited, boilers On the other hand, in steam heating attack is very slight. ^ is frequently suuulied, corrosion or range boilers to which fresh water retards it. can be progressive unless scale formation Americ an Society f Submerged Water Corrosion Tests Made by the ^ Testing Materials conducted, from 1920 The American Society for Testing Materials corrosion tests of ferrous metals. Since the con- to 1930 , submerged water storage tanks, the ditions were somewhat anaiagous to those in summarized here. method and conclusions of this test are briefly representing com- Samples of uncoated sheets (16 and 22 gage) added copper, mercially available and , with and without which water was passed. were set up in long narrow boxes through toapolis, Md. At Washington, W. C., tap water was used and at cover the Severn River water passed. A water level sufficient to of flow was such t a sheets at all times was maintained, and the rate sheet was there was comulete change of water every I 6 minutes. A more placeji. The considered to .have failed when perforated at one or shown 'in characteristics of the waters affecting corrosiveness are Table 1 and the results of the test in Table 2. lasted much As shown in Table 2, the 22 and 16 gage sheets gage sheets) longer at Annapolis (5S uercent longer life for the 22 Table than at Washington, although the water at Annapolis (see 1), the was more highly contaminted with suluhates and chlorides than Washington water. The conditions of corrosion, however, were such forma.- that' metal attack uroceeded at Washington without appreciable was true tion of protective scale on the sheets, whereas the reverse the more at Annapolis. This factor comuensated in large measure for an corrosive nature of the Annapolis water. The test is, therefore, self-formed illustration of the importance, in retarding corrosion, of Washington protective of this kind. It will be noted, both at with and Annapolis, that the kind of ferrous metal used, whether made under or without copper, was of secondary importance to the conditions which corrosion took place. This latter condition is believed toehold generally for submerged corrosion and is in contrast to atmospheric more service, for which steels or irons with added copper are considered durable, (see TIBM No. 10).

- .3 - 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,

coitr- 3 , In the "submerged" corrosion of steels and irons, the 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 advanta- geous in submerged corrosion service.

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