UCRLJC-122736

Water Vapor Effects On The Corrosion Of Steel

This paper was prepared for submittal to the 1996 International High Level Radioactive Waste Management Conferkce

Las Vegas, NV April 29 - May 3,1996

November 16,1995 . . . . .. .

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This is a preprint of a paper intended forpublication in a journalorproceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.

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DISCLAIMER

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WATER VAPOR EFFECTS ON THE CORROSION OF STEEL John C. Estill and Gregory E. Gdowski

Lawrence Livermore National Laboratory

INTRODUCTION

The effects of water vapor partial pressure on the corrosiodoxidation behavior of

carbon, low alloy, and cast steel in the temperature range of 50 - 150 "C at atmospheric

pressure were investigated using thermogravimetric analysis. At temperatures above the

boiling point, which is dependent on the presence of hygroscopic salts and surface

condition, the interest is in the effect of water vapor on dry oxidation. At temperatures

below the boiling poirit, where water condensation may exist, the interest is in the

presence of thin water films in which electrochemical processes may occur. The presence

and corrosivity of thin water flms are dependent on temperature, water partial pressure,

surface condition, gaseous constituents, and adsorbed hygroscopic salts.

These studies were conducted in support of the Yucca Mountain Site

Characterization Project (YMSCP) and are part of the characterization of the behavior of

metallic materials in expected environments at a potential high-level radioactive waste

repository at Yucca Mountain, Nevada. The potential repository is above the water table,

and waste packages will be exposed to elevated temperatures and potentially high water

partial pressures. The materials tested in this study are being considered as an outer barrier

material in a multi-barrier containment concept. Inner barrier materials would be

constructed of a more corrosion resistant material such as a titanium based alloy, Ni-Cr-

Mo or Ni-Fe-Cr-Mo alloy. The focus of this work is to determine the corrosiodoxidation

performance of the outer barrier material.

EXPERIMENTAI-4

The experiments were conducted in a modfied thermogravimetric analyzer (TGA)

which continuously measures the weight change of a specimen while it is undergoing

oxidation or corrosion processes in the presence of various partial pressures of water. The

weight measurement resolution of the TGA is approximately 60 pg.

The temperature of the reaction chamber is controlled by circulating fluid through

an annular chamber formed by a double-walled glass cylinder. The fluid, which is

continuously circulated in a closed loop between the annular chamber and a temperature- -

controlled bath, provides a temperature deviation of less than 0.5 "C along the length of

the specimen contained inside the inner wall of the glass cylinder (reaction chamber).

The relative humidity is varied in the reaction chamber by mixing precise

amounts of vaporized water and dry air. The vaporized water is provided by a

vaporizerhicrornetering pump assembly, and the dry air is supplied using a mass flow

controller. Controlled mixing of the vaporized water with the dry air is the means used to

vary the relative humidity.

Test specimens measured 3.8 cm x 1.3 cm x 0.16 cm. The nominal weight of a

specimen is 5.6 grams. The specimens were suitably degreased with isopropanol prior to

testing.

The test parameters of time, temperature, weight, and relative humidity were

recorded and stored electronically.

RESULTS

Preliminary results for AISI 1020 carbon steel specimens exposed to a range of

humidified environments at 65 "C were consistent with aggressive oxidation at 2 85% RH

and little oxidation at and below 75% RH (see Figure 1). The aggressive electrochemical

corrosion processes at or above 85% RJ3 exhibit rapid corrosion initially but slow

dramatically after the first 24 hours.

These results indicate that in the range of 75-85 % RH the transition from dry

1- oxidation to electrochemical corrosion is occurring. The critical relative humidity value in

this range is similar to that observed for carbon steels at ambient temperatures. 1

Visual examination of those specimens exposed to 85% RH showed uniform

reddish-brown corrosion product present on the surface, while those specimens exposed

to lower relative humidity were virtually non-oxidized. Examination conducted with an

optical microscope at higher mamcation revealed some areas of discontinuous attack on

the specimen exposed to moist air at 85% RH. Also, there was some reddish-brown oxide

present on the edge surfaces of the coupons exposed at 65 and 75% RH. In all cases the

reddish-brown corrosion product was non-adherent, and an underlying black oxide layer

was present.

CONCLUSIONS AND DISCUSSION

The critical relative humidity for AISI 1020 carbon steel is between 75 and 85%

RH at 65 O C . Electrochemical corrosion occurs at higher relative humidity (X5 %m, while dry oxidation occurs below 75% RH. Although no ve-g chemical analysis was

conducted on the corrosion product, a review of the CRC handbook indicates that the

reddish-brown corrosion product could be either Fe03*xH20 or Fe203. The color of the

black oxide layer is consistent with that of Fe304.

The difference in corrosion behavior of two differing surfaces on the same

specimen (the face surface and the mill-machined edges) shows the dependence of surface

finish on corrosiodoxidation behavior The samples exposed at 65 and 75% RH exhibited

notably different results; that is, the face surfaces had little or no oxidation or corrosion

products, while the mill-machined edges were corroded with non uniform areas of reddish-

brown corrosion product.

6-1

5 -

n

E" 4 - Y c

m m

- 6 3- z

2- 5

I /

I I I 0 2: 40 60 80

Time (hr)

Figure 1. Weight gain of AIS1 1020 carbon steel specimens as a hc t ion of time in atmospheres of air at various relative humidities at 65OC.

REFERENCES

1. Mor, W. J., ed., Atmospheric Corrosion, John Wiley & Sons, New York, 1982.

2. CRC Handbook of Chemistry and Physics. 75th Edition, D. R Lide, Editor-in- Chiefl CRC Press, Boca Raton, FLY 1941, p. 4-66.

*This work was performed under the auspices o f the U.S. Depdrtment of Energy by Lawrence Livermore National Laboratory .under c o n t r a c t No. W-7405-Eng-48.

Technical Inibrniafion Department e Lawrence Livermore National Laboratory University of California . Livermore, California 9455 1

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