I
by Gordon P. K. Chu Pfaudler Permutit Co.
E Q U I P M E N T AND A
W O R K B O O K
F E A T U R E
DESIGN
E C
Ρ 1J
Hydrogen Behavior of Metals — A Factor in Selection of Metals for Equipment Fabrication Hydrogen absorbed in steel can later cause coatings on equipment to blister or chip I ROPER selection of specific types of metal for t h e construction of struc-
tures or equipment is of great impor tance to the fabricator and user. I t is generally based on chemical com position, tensile strength, yield point, a n d elongation. Requirements a r e rarely specified in terms of t h e hy drogen behavior of the metals. Yet, during t h e last decade, t h e problem of hydrogen in steel has aroused great attention from t h e point of view of industrial application. T h e chemical and chemical proc
m Glass coating on mild steel weld• metal chipped completely (bot tom). Chipping is caused by excessive accumulation of molecular hydrogen at metal-glass surface. When steel is molten, its capacity for absorbing .hydrogen can be 1000 times greater than at room temperature. On fast cooling, the steel, being supersatu rated with the hydrogen, has to pre cipitate the excess dissolved hydrogen (in atomic form), to approach equilib rium—hence the chipping. Condi tion can be prevented (top) by proper treatment, such as depleting the hydrogen in the steel before the enam eled steel is cooled to room tempera ture or surface alloying with materials not permeable to hydrogen.
w
How Hydrogen Absorbed in Steel during Manufacture Can Cause Headaches Later on in Equipment Made from the Steel
•
1
essing industries, particularly those involved in oil refining a n d hydro génation (6), have suffered great losses from t h e destruction -due to the cracking or blistering of steel. Coating industries, organic or inorganic, metallic or nonmetallic (3), have also encountered chipping, blistering, a n d peeling as a result of hydrogen liberation from steel. Even steam boilers have m e t with shortened life. Various physical and
0
Excess hydrogen in steel coated with glass on both sides causes glass chipping. Photograph on top shows chipped condition. Note chipping occurred only where glass was applied to both sides of the steel. Bottom, chipping was prevented on both sides by depleting the hydrogen in the enameled metal.
I/EC
0
Acid spilled on top of an exposed steel head of a glass tank may cause chipping. The reaction: Fe2 + H 2 S 0 4 -»- FeSO., + 2(H). The atomic hydrogen, H, is very penetrating to steel, and contamination of any acid with base steel must be avoided. In case of leakage or spill, the contaminated area should be flushed with water. Where circulating water in the jacket of a glass-lined vessel is acidic, an acid-steel reaction may lead to failure of glass coating on the opposite inner side of the tank. When the pH goes below 5, the water should be treated to bring the pH up.
WORKBOOK FEATURES
59 A
I/EC
EQUIPMENT A N D DESIGN
Liquid steel Melting point
Figure 1. As temperature and pres sure go up, so does the solubility of hydrogen increase
15 cm.
Gasket
Figure 2. Electrolytic cell for hydro gen diffusion through metal specimens, S, by cathodic charging
Mild steel ISAEIOIOK
no treatment leatment
/
Cr treated mild steel 'Ni treated » ....,,, , , mild s t e e [ ) l _ - s = ^ ' 7 3 0 T s ainless steel
Time.hrs.
Figure 3. Different rates of hydrogen diffusion through different 1 4 - g a g e test pieces
chemical properties of solid ma terials, including metals as well as glasses, can be explained a n d under stood from the viewpoint of crystal structures (2, 4). Figure 1 illustrates the effect of temperature and pressure upon the solubility of hydrogen in iron (5, 7). H i g h temperature mild steel upon fast cooling, often results in supersaturation of hydrogen which will be precipitated out either in the metal voids or between the coatings a n d steel, resulting in destruction. 60 A
•
A Workbook
Feature
Experiments in the research lab oratory of T h e Pfaudlcr Co. have been centered a r o u n d hydrogen characteristics of steels and alloys. T h e sources of hydrogen entering into steel have been traced and the mechanism of hydrogen absorption, diffusion, a n d evolution has been studied. T h e case histories in the box (page 59 A) are typical examples illustrated by the chipping of glass on mild steel, precipitation of hydrogen under different circumstances. H o w ever, the troubles can be brought under control by the application of theories. T o evaluate the solubility and dif fusion behavior of hydrogen in vari ous metals, the author devised a special hydrogen diffusion cell. Metals in sheet form having the same thickness are assembled in a simple electrolytic cell (Figure 2). W a t e r is actually electrolyzed into hydrogen and oxygen by a direct current, using dilute sulfuric acid as an electrolyte. Steel sheets are used as two cathodes, one at each end of the cell. A platinum spatula is used as anode at the center. W h e n hy drogen is nucleated at the steel cath odes, a small part of the hydrogen (nascent) enters into the metal by the process of diffusion. As a result, a thin plate of steel needs only a few minutes to have hydrogen penetrated through. If one side of a steel sample sheet is glass coated, the glass will chip after only a short period of cathodic charging of hydrogen from the opposite bare metal side, where nascent hydrogen is continuously nucleated. Different kinds of glass or other metallic coatings on steels, as well as other base metals, have been extensively tested. W h e n bare metal sheets are used as test specimens {S, Figure 2), the electrolytic cell collects the diffused hydrogen in separate burets. Cu mulative hydrogen ( n u m b e r of cubic centimeters permeable through metal sheets) is recorded as a function of time. Different metals, or the same metal with differing treatments, re sponded sensitively toward hydrogen permeation. Figure 3 shows the rates of hydrogen diffusion of several materials. Mild steel is most sus ceptible to hydrogen diffusion among the group. T h e hydrogen permea tion through the nickel or chromiumtreated mild steels is slowed down be cause the rate of diffusion is reduced,
INDUSTRIAL AND ENGINEERING CHEMISTRY
as shown by the retarded slopes of the curves. T h e initial time (intercept on the abscissa of Figure 3) required to " p u s h " hydrogen through the steel and to cause nucleation of bubbles on the opposite side of a test specimen in the electrolytic cell is greatly in creased. T h e magnitude of the t i m e intercept indicates the initial hydro gen solubility in steel. Stainless steels and Hastelloys are virtually nonpermeable to cathodic hydrogen under the conditions tested. Generally, in materials of similar type, such as the mild steel group, the ease of passage of atomic hydro gen t h r o u g h a metallic lattice is directly correlated to the suscepti bility to corrosion—i.e., the metals easily permeable to hydrogen arc least resistant to corrosion. How ever, mild steel, after controlling of processing steps, such as proper heat treatment, surface alloying, protec tive atmospheric firing and cooling, can be used with great assurance. T h e improvements to the resist ance of the surface-alloyed steels, low-alloy steel, and special alloys toward hydrogen diffusion are marked. W i t h a basic knowledge of the hydrogen behavior in metals, a n d a proper method of evaluation, one can predict with some assurance how certain materials in a particular en vironment will respond, and thereby reduce or completely avoid trouble caused by hydrogen.
References (1) Am. Soc. Metals, Cleveland, Ohio, " M e t a l Handbook," 1948. (2) Barrett, C. S., "Structure of Metal," McGraw-Hill, New York, 1955. (3) Chu, G. P. Κ., Keller, J. H., Davis, H . M., J. Am. Ceram. Soc. 36, 48-59 (1953). (4) Goldman, J. E., "Science of Engi neering Materials," Wiley, New York, 1957. (5) Martin, Erich, Arch. Eisenhiittenw. 1929-30, 407-16. (6) Nelson, G. Α., Effinger, R. T., Welding J., 34, Research Suppl., 1-11 (1955). (7) Sieverts, Α., Ζ. physik. Chem. 77, 591-613 (1911).
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
