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A system for monitoring hydrogen-metal reaction kinetics
241
Journal of the Less-Common Metals, 49 (1976) 241 - 252 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
A SYSTEM KINETICS*
FOR MONITORING
S. C. LAWRENCE, Lawrence
Electronics
HYDROGEN-METAL
REACTION
Jr. Co., 14636
Ambaum
Blvd.
SW, Seattle,
Wash. (U.S.A.)
J. C. PICARD Mktaux
Inoxydables
Ouvres,
42, Au. Grande
Arm&e,
Paris 75017
(France)
R. REICH Laboratoire
de Physique
des Solides,
(Received February 7, 1976
Universitk
Paris-Sud,
91504
Orsay-Cedex
(France)
1
Summary The early use of metal radio tubes as ion gauges for monitoring hydrogen permeation through the metal, is reviewed. The development of improved manufacturing and procedural techniques, together with the development of an integrated system of cleaning, rinsing and electronic heating of the hydrogen probe, has provided a convenient method of hydrogen measurement. The problems of information-loss due to probe ion pumping (get&ring) have been corrected by integrating the hydrogen pressure curve with respect to time. Hydrogen-permeation membranes of 1010 steel and of palladium-silver 75/25%, have been used to control electroplatingbath characteristics, to measure corrosion, and for failure analysis. An examination of some of the heterogeneous equilibrium reactions of the measure ment system is made from both theoretical and operational points of view, in order to explain the probe dynamics.
History F. J. Norton [l] was one of the first investigators to use a commercial vacuum tube, viz., a steel-shelled 6C5, to measure hydrogen reactions resulting from corrosion, and electroplating accompanied by corrosion. He measured the hydrogen pressure inside the tube as a function of the grid current of the tube. In 1956, McCaldin [2] increased the measurement sensitivity by a factor of X 50 by using a 6V6 beam-powered tetrode, with the *Presented at the Meeting on “Hydrogen in Metals”, held at the University of Birmingham, United Kingdom, on January 5 - 6, 1976, under the auspices of the Chemical Society (Faraday Division).
242
HYDAOGEN/CORROSION
-STEEL SHELL
-HYDROGEN ~~~~~4TRATION OUTGASSING ICATHiDE
(0'4)
+HYDROGENPROTON "'m""ELECTRONFLOW
-HYDROGENATOM -HYDROGEN MOLECULE
Fig. 1. Hydrogen probe measurement, schematic.
screen grid as accelerator, and the plate as ion collector. A lack of reproducibility of the results using the same probe and between different probes led to abandonment of the system.
The hydrogen sensor, or probe, measures the hydrogen pressures by ionizing the internally desorbed hydrogen molecules with a beam of electrons emitted by a filament-heated cathode. The ions so formed (Fig. 1) are collected on a negatively-charged ion plate. By measuring the neutralization current flowing to this plate, one ha8 a measure of the probe-vacuum between 5 X 10e8 and 10d3 Torr. Below 5 X 10P8 Torr,.the current produced by photoemission from the plate, due to soft X-ray radiation from the probe filament, masks other currents. We have made a study of some of the unresolved variables experienced by Norton [l] and McCaldin [ 21 and provided the necessary corrections. These included standardization of the steel used for the hydrogen window, and a closer control of the electrode spacing, and of other manufacturing processes. The result was a hydrogen probe rather than a radio tube. Even so, it was necessary to develop calibration techniques so that the results with different probes could be normalized to give the same reading for the same hydrogen exposure.
Calibration A standard amount of hydrogen was electrolytically deposited on a given hydrogen window (Fig. 2) that had been activated by controlled abrasion.
243
Fig. 2. Probe hydrogen window size.( x 314) Fig. 3. Hydrogen/corrosion measurement gauge, H/CMG-5A. YANGESW\TCHlN:
t
o-;;OBE
j-30 SEC-; INTEG.
90 SECIiTEG. (PROBEINTOOVEN)
TIME-
Fig. 4. Probe hydrogen plateaux under equilibrium conditions.
A rinsing system with controlled pressure and temperature was used to remove electrolytes in a reproducible manner. Afterwards, the probe was dried and heated rapidly to a controlled temperature in a conforming oven in the hydrogen gauge. The gauge (Fig. 3) was also provided with appropriate voltages and currents so that the hydrogen pressure curve could
244
be recorded (Fig. 4). I,,,, the integral of that curve to the hydrogen pressure peak, HP, is electronically displayed, as is the hydrogen pressure peak, and h, the time in seconds from HP to HP/Z. Tile calibration pressure curve rises to a hydrogen pressure peak, HP, which is divided by 100 to give a “HP calibration factor”; future probe hydrogen exposures are divided by this factor in order to normalize them. The time in seconds, measured from switching on the probe to the hydrogen peak (HP) is called y. The measured integral of the pressure curve, from switching on to hydrogen peak, is I,, and the normalizing 1, factor is 1,/100. In the case of h (defined above), the normalizing factor is X (measured) 140. The use of these dividing factors normalizes the measurements made with different probes. Clean-up Hydrogen is completely removed from the probe before each test by electronically heating the probe while ionizing the hydrogen; this is referred to as clean-up or CU. The probe hydrogen pressure during outgassing can be monitored by using the hydrogen window as an ion collector. This permits precise and reproducible starting pressures to be obtained. After CU, the probe is cooled for at least one h,our prior to re-exposure. If the re-exposure is a second calibration, the measured I7 is closely reproducible. The HP increases by approximately 6% with each succeeding calibration, while h decreases at about the same rate. These effects are due to the decrease in the steel membrane thickness caused by the abrasion, accompanying surface activation. Probe reaction
kinetics
A knowledge of the heterogeneous equilibrium reactions which accompany probe operation is of value for the understanding of probe hydrogen reaction kinetics. Probe pressure differences, by themselves, do not give an accurate measurement of absorbed hydrogen in the system, because of hydrogen pumping (gettering~ and release. The differenti~ equation repre senting these reactions is: dP/dt = dH/dt - dG/dt
+ dlYJdt
where dP/dt is the observed pressure change with time, dH/dt the rate of hydrogen input from the shell, dHJdt the rate of hydrogen recirculation from collector plate and barium getter, and dG/dt is the rate of all pumpings, equals UP, where a is the pumping speed. The ion pumping speed is 1 probe volume per second at 6 mA ionizing electron current. The pumping by the barium getter used to produce the initial high vacuum is comparable; both reactions are reversible with increased
245
temperature; the release of hydrogen is proportional to the hydrogen absorbed and is exponentially dependent on the temperature. The Ba is located on the inner surface of the probe metal shell and its temperature will follow that of the shell, approaching 140 “C under oven conditions. The ionization plate will approach 140 “C! more slowl.y, due to its physical separation from the shell. Heat-transfer will be primarily due to radiation, since, at the low probe pressures, convection is small. The recirculation of hydrogen, by the plate and getter, produces an overall pressure decay curve that is not truly exponential. It is actually the difference of two exponentials; one due to a hydrogen permeation through the hydrogen window and the other due to recirculation effects. Where the hydrogen exit window is a combination of steel and palladium, as in earlier Pd-Ag probe construction, we observe that the log decrement of the decay rate is not equal for equal times but, on the contrary, that each succeeding time interval must be multiplied by a constant. At a temperature of 250 ‘C, in an atmosphere of heated air, this constant multiplier, for an initial 20 min increment, was found to be 1.2. The decay observed was 20% per increment and the relationship was maintained over 24 h. Since the major contribution to the probe pressure following HP has been found to be due to recirculation effects, integration of the hydrogen pressure curve to its peak, HP, has been found to be proportional to the hydrogen absorbed. Integration also provides a significant improvement over earlier methods where the HP alone was used. The measured value of HP is critically dependent on the exact oven temperature, the probe insertion into the oven, and the thermal environment of the oven. The integral to the peak (I,) is much less sensitive to these effects.
Experimental
equilibria
One can demonstrate equilibria of another kind by depositing hydrogen at different current densities and from different electrolytes (Fig. 5) and balancing this input with hydrogen desorption from the probe. When the solution was 0.01 M H,S04 and the current density 0.04 A dmw2, the hydrogen pressure plateau, representative of equilibrium conditions, at 5 X lo-’ Torr, was reached after 2$ h. When the hydrogen was deposited at 2 A dme2, a level of 3 x lop6Torr was reached after an additional hour of charging. Further increases in current density in this electrolyte created hydrogen bubbles which masked the surface and, in this case, the proportion of hydrogen absorbed by the probe was decreased. This effect is not observed, even at 8 A dmp2, when hydrogen “poisons”, such as cyanides, cover the surface and change the ratio between adsorbed hydrogen and recombined hydrogen [ 31. Agitation of the electrolyte also removes the blocking bubbles and increases sorption. Increases of temperature increase the proton diffusion rate. This removes adsorbed hydrogen ions from the surface and leaves a surface-active “vacancy” for readsorption of another hydrogen atom. Both factors increase the ratio of hydrogen sorbed to hydogen lost to molecule formation.
246 0T NON-EQUILIBRIUM
HEAVYBUBBLEFORMATION (SURFACE OCCLUDED)
TIME(HOURS) Fig.
5. Probe HP, h, 7, Zy, and I,
curve (ASD = A dm-‘).
Probe analog A simple analog to the probe hydrogen sorption and internal hydrogen pumping is obtained by visualizing the probe as a funnel, into which the hydrogen is introduced and from which it disappears at a rate proportional to pressure levels (Fig. 6). The probe pressure corresponds to the “depth” of the hydrogen in the funnel. At equilibrium, the pressure remains constant for each hydrogen input within the pumping capabilities of the probe. The equilibrium pressure may be increased or decreased as a function of the hydrogen input. The ion pumping by the collector may be turned off by turning off the probe electronically. The barium-getter pumping is reduced by lowering the hydrogen sorption temperautre. For example, the getter pumping is lowered more than 30-fold by making the test exposure at 20 “C rather than at 100 “C.
Measurement
of high hydrogen
inputs
The pumping may be used effectively for monitoring relatively high hydrogen input rates. This has been used for following the hydrogen sorption by steel during chromium plating at 50 ‘C, leaving the probe on. In this case, the equilibrium pressure level was maintained by two factors, i.e., internal hydrogen pumping and reverse hydrogen permeation through the steel window.
247 PROBE
FUNNEL ANALOG
PROBE OFF
ION Cp~m~CTOR
t!IN
."'+ BARIUM GETTER
H IN H' IN = H' OUTPROBE ON
* HYDROGEN PROTON
Fig. 6. Probe
funnel
'HYDROGEN ATOM
-HYDROGEN MOLECULE
analog.
These rates are proportional to internal hydrogen pressure. The latter effect, no doubt, increases the proportion of plated hydrogen that bubbles away, since it would tend to fill hydrogen-active sorption sites from the reverse side and thus lead to recombination reactions at the surface. The response time of the system is very rapid. With a steel window 0.4 mm thick, new equilibrium levels as a function of temperature and current density changes were established within minutes.
Steel window probes: plating In early techniques used for the control of low-embrittlement cadmium plating, a steel probe having a well-defined steel hydrogen window outlined by a special epoxy paint (Fig. 7), was surface-activated by abrasive blasting, plated to full specification thickness, rinsed, dried, turned on, and inserted into the small encircling oven of the hydrogen gauge. A hydrogen pressure curve us. time was obtained (Fig. 4). The important parameter empirically selected for control was h, the time between HP and HP/2, and this was
248
Fig. 7. Epoxy-masked steel hydrogen probe.
found to vary with the permeability of the applied plate of low-embrittlement cadmium plating. This plate permeability v&s of critical value for determining the time required to bake plated parts to remove hydrogen to safe levels. However, we encountered problems when we tried to apply the above techniques for the control of brightener levels in a copper pyrophosphate bath, since we had a relatively impermeable hydrogen barrier. Empirically, we were led to the “time-lag” method of determining the diffusion coefficient of the plate [4] . The data obtained gave a highly reproducible parameter, y, which varied with brightener concentration. As more brightener was added, the crystal structure of the electroplate became finer, and the hydrogen diffusion coefficient decreased as the plating became more impermeable to hydrogen. Many years have been spent in developing commercial X standards that correlate with hydrogen embrittlement thresholds in sustained load specimens of 4340 steel, heat-treated to 260 - 280 ksi, ultimate tensile strength. However, since the standard A deviation exceeds 100% for h greater than 120 s, early conversion to y standards will require either complete re-correlation studies to be made directly, or else some method of conversion between the two will have to be found. The gauge has proved useful in studying the difference between lowembrittlement and bright (high-embrittlement) cadmium-plating systems. In the former, most of the hydrogen deposited goes directly into the steel. Dissolving the plating in NH,NOa before inserting the probe into the oven, produces HP values which are almost identical with those measured without removal of the plate.
249
*+/ ‘:
STEEL-
4
;;L-d d,+ *
+ fi + I '+ ,;
lOOiO?At 90 I STEEL.CAOMhl
“I
.w I
J,’ Y ‘*/,
*,
/+, .: ,'*,',* ',, (+ ,/ '+ ,'.' I"Tr?Tlls / t + 101 STEELCAOMlhi ' '+,
I”
IYl”L
I
IO 1
5
’‘, /.!I ‘, ’ .‘!/ \ ’
",
/ ,
,
,,, , , / / / ' / ,',' IM :' oioSTEE~.CAOMIL , ,'/
100
100
L1
END OF BAKE
MID BAKE
AFTERPL, ATINGBEFOREBAKING
TIME(H6URS) 23
+ HYDROGEN PROTON
Fig. 8. Hydrogen
0 HYDROGEN ATOM
location
AFTERPLATINGBEFORE BAKING
m HYDROGEN MOLECULE
in low-embrittlement
cadmium-plated
MID BAKE
END OF BAKE
steel during baking.
FINE GRAINED CADMIUM STEEL5OOdTOTAt.' /, IOOO-1STEEL. CADMIUM
10.000 -
1000~1 STEEL CAOMIUM~"
5000
%
5
TIME(HOURS)23
+ HYOROGEN PROTON
Fig. 9. Hydrogen
0 HYOROGEN ATOM
location
in bright
(dense)
-HYDROGEN MOLECULE
cadmium-plated
steel during
baking.
In the case of bright (fine-grained) cadmium plating, the situation is quite different. Most of the hydrogen is still in the plate at the end of the electroplating time. If the plating is dissolved before placing the probe in the oven, the hydrogen input is reduced to only 1% of the value observed when the plating is left in place during the probe heating. This shows that, contrary to low-embrittlement practices, baking bright cadmium-plated parts increases, rather than decreases, the average hydrogen content of the steel (Figs. 8 and 9) [ 51. Confirmation is found in the work of Kudryavtsev etal. [6]. Steel window probes:
stress corrosion
For many years, low-embrittlement cadmium-plated steel probes have been used to measure hydrogen sorption from solutions that may, in service, contact similarly plated high-strength steel parts under stress. The use of the probe-heating step, which does not, in this case as in plating, simulate treatment of plated stress-rupture parts, has presented problems. Variations
250
exceeding 100: 1 in the time to failure of the stressed plated specimens themselves have added to the general confusion regarding correlation. Present techniques consist of exposing the probe for 24 h to the solution under test. This, since 1960, has provided an increased degree of protection to the airlines. This exposure time has proved satisfactory, contrary to tests made at one hour or 45 min; the latter cannot distinguish between safe, moderately safe, and embrittling fluids. There is still no acceptance of a single general test procedure. The difficulties associated with heating the probe after test exposure, are that, at that time, part of the hydrogen measured comes from the cadmium plate and part from the steel. Yet, in stress corrosion correlation studies, this kind of measurement has been compared with plated, exposed, stiess-rupture specimens that are sensitive only to hydrogen in the steel. These are not baked, like the probe, to drive hydrogen from the plate inwards. A solution to this problem is achieved by measuring probe pressure levels after test, prior to oven baking. This is done by measuring the probe pressure maximum between 30 and 60 s inclusive, immediately following rinsing at 50 “C and drying. This value can also be obtained by integrating the area under the hydrogen pressure curve between the two times. The test simulates maximum expected field-temperature conditions.
Palladium-silver
probes:
corrosion
and post plating measurements
Development of probes having a Pd-Ag, 75%/25% hydrogen window instead of steel, permits the gauge to be used for monitoring molecular hydrogen in liquids and gases. The competing reaction of hydrogen with oxygen to form water on the Pd-Ag surface is avoided by limiting the exposure to atmosphere to a maximum of 20 s. The heating is in an inert gas, such as argon, that has been selected to have a maximum of 10 ppm impurities, with 0 ppm H, or hydrocarbons. Hydrogen Pd-Ag window surface activation is both chemical and mechanical. The amount of hydrogen introduced during this step in the procedures is small, yet it provides effective competition for the filling of active sites on the Pd-Ag with hydrogen, instead of oxygen trapping prior to test exposure. The chemical is sodium hypochlorite and the polishing of the water-wet Pd-Ag surface is carried out with abrasive powders. Initial work with this probe was carried out by the United States Navy to see if it was possible to correlate probe molecular hydrogen sorption from aqueous corrosion reactions with tests such as salt spray, salt water, acetic acid tests, and EXFO (exfoliation susceptibility tests of different heat treatments of aluminum alloys) [7]. The results obtained by immersing the probe in the corroding solution for 15 min showed excellent correlation with the older, visual examination techniques. This technique is particularly suited to the Pd-Ag window because it uses the high hydrogen solubility in Pd-Ag at room temperature to act as a sponge, and the high diffusion rate above 250 “C to increase system response time and sensitivity.
253. TABLE
1
Hydrogen concentration Time (min.)\
in high-strength steel us. extraction
time and post-plate bake time
Condition
30 60
I
II
8.1 306
52
1.8
III
IV
0.1 2
34
1.6
V
VI
0.2
0.01
4.7
0.3
Additional applications have been made for measuring residual hydrogen levels in electroplated, high-strength steel after low-embrittlement cadmium plating. After the parts were processed by the following procedures, they were analyzed. The samples were enclosed in a 1500 ml chamber under a cover of 99.999% pure argon. The chamber was pressurized to 2 bar and returned to 1 bar 11 times, reducing the oxygen content by a factor of 103. The sample was then heated at 300 “C in an aluminium crucible (hydrogenfree metal). The Pd-Ag probe was then heated for 5 min at 250 “C and the equilibrium hydrogen pressure measured electronically. Allowances were made for hydrogen loss due to various probe reactions: I II III IV V VI
Acid activated + Cd plate + no bake. Acid activated + Cd plate + 3 h bake at 200 “C. Acid activated + Cd plate + 23 h bake at 200 “C. Sandblasted + Cd plate + no bake. Sandblasted + Cd plate + 3 h bake at 200 “C. Sandblasted + Cd plate + 23 h bake at 200 “C.
Table 1 lists the ppm of hydrogen found in each class of procedure after 30 and 60 min of heating at 300 “C. The amounts shown constitute only a part of the total adsorbed during the plating process. To obtain total hydrogen, one would have to remove the Cd barrier and heat the samples at higher temperatures. Distinct differences are observed between different sample classes and for extended bake times. The tests were made several months after the parts were plated. It is seen that the residual hydrogen in the acid-activated samples is nearly 10 times higher than that of the sandblasted ones. The results would have been higher for the acid samples had not their hydrogen effusion rate been reduced by darkening. It was observed that significant sample darkening occurred with the acid-treated samples. The effect of the plated surface barrier on hydrogen-loss measurements should not be overlooked. In the case of hydrogen analysis in a bright Znplated, high-strength steel hose clamp (Fig. lo), only 1 ppm of hydrogen was measured after 3 h bake at 200 “C. The sample was broken, exposing the unplated surfaces. On reheating, a hydrogen level of 1000 ppm was obtained in 10 min, as the hydrogen was then able to escape to the front from the unblocked samples.
Fig. 10. Zinc-plated
(embrittled)
high-strength
steel hose clamp.
Conclusion Past me~u~ements have indicated a number of different plating and corrosion parameters that may be studied with the hydrogen gauge and probes. More rapid hydrogen-content testing of samples has been made possible by induction heating, Using glass instead of metal for the Pd-Ag envelope improves the sensitivity so that hydrogen partial pressures of the order of low8 Ton can be measured.
References 1 2 3 4 5 6
F. J. Norton, J. Appl. Phys., 11 (1940) 262. J. 0. McCaldin, J. Appl. Phys., 27 (1956) 307. W. Beck, J. Electrochem Sot., 112 (1965) 53. R. M. Barrer, Trans. Faraday Sot., 36 (1940) 1235. S. C. Lawrence, ASTM 543,83, Hydrogen Detection Gage, presented V. N. Kudryavtsev, K. S. Pedan and A. T. Vagramyan; Electrodepos. (1972/73) 213. 7 P. Fischer, Report No. NADC-72251.VT, 1972.
June, 1972. Surface Treat.,
1
Journal of the Less-Common Metals, 49 (1976) 241 - 252 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
A SYSTEM KINETICS*
FOR MONITORING
S. C. LAWRENCE, Lawrence
Electronics
HYDROGEN-METAL
REACTION
Jr. Co., 14636
Ambaum
Blvd.
SW, Seattle,
Wash. (U.S.A.)
J. C. PICARD Mktaux
Inoxydables
Ouvres,
42, Au. Grande
Arm&e,
Paris 75017
(France)
R. REICH Laboratoire
de Physique
des Solides,
(Received February 7, 1976
Universitk
Paris-Sud,
91504
Orsay-Cedex
(France)
1
Summary The early use of metal radio tubes as ion gauges for monitoring hydrogen permeation through the metal, is reviewed. The development of improved manufacturing and procedural techniques, together with the development of an integrated system of cleaning, rinsing and electronic heating of the hydrogen probe, has provided a convenient method of hydrogen measurement. The problems of information-loss due to probe ion pumping (get&ring) have been corrected by integrating the hydrogen pressure curve with respect to time. Hydrogen-permeation membranes of 1010 steel and of palladium-silver 75/25%, have been used to control electroplatingbath characteristics, to measure corrosion, and for failure analysis. An examination of some of the heterogeneous equilibrium reactions of the measure ment system is made from both theoretical and operational points of view, in order to explain the probe dynamics.
History F. J. Norton [l] was one of the first investigators to use a commercial vacuum tube, viz., a steel-shelled 6C5, to measure hydrogen reactions resulting from corrosion, and electroplating accompanied by corrosion. He measured the hydrogen pressure inside the tube as a function of the grid current of the tube. In 1956, McCaldin [2] increased the measurement sensitivity by a factor of X 50 by using a 6V6 beam-powered tetrode, with the *Presented at the Meeting on “Hydrogen in Metals”, held at the University of Birmingham, United Kingdom, on January 5 - 6, 1976, under the auspices of the Chemical Society (Faraday Division).
242
HYDAOGEN/CORROSION
-STEEL SHELL
-HYDROGEN ~~~~~4TRATION OUTGASSING ICATHiDE
(0'4)
+HYDROGENPROTON "'m""ELECTRONFLOW
-HYDROGENATOM -HYDROGEN MOLECULE
Fig. 1. Hydrogen probe measurement, schematic.
screen grid as accelerator, and the plate as ion collector. A lack of reproducibility of the results using the same probe and between different probes led to abandonment of the system.
The hydrogen sensor, or probe, measures the hydrogen pressures by ionizing the internally desorbed hydrogen molecules with a beam of electrons emitted by a filament-heated cathode. The ions so formed (Fig. 1) are collected on a negatively-charged ion plate. By measuring the neutralization current flowing to this plate, one ha8 a measure of the probe-vacuum between 5 X 10e8 and 10d3 Torr. Below 5 X 10P8 Torr,.the current produced by photoemission from the plate, due to soft X-ray radiation from the probe filament, masks other currents. We have made a study of some of the unresolved variables experienced by Norton [l] and McCaldin [ 21 and provided the necessary corrections. These included standardization of the steel used for the hydrogen window, and a closer control of the electrode spacing, and of other manufacturing processes. The result was a hydrogen probe rather than a radio tube. Even so, it was necessary to develop calibration techniques so that the results with different probes could be normalized to give the same reading for the same hydrogen exposure.
Calibration A standard amount of hydrogen was electrolytically deposited on a given hydrogen window (Fig. 2) that had been activated by controlled abrasion.
243
Fig. 2. Probe hydrogen window size.( x 314) Fig. 3. Hydrogen/corrosion measurement gauge, H/CMG-5A. YANGESW\TCHlN:
t
o-;;OBE
j-30 SEC-; INTEG.
90 SECIiTEG. (PROBEINTOOVEN)
TIME-
Fig. 4. Probe hydrogen plateaux under equilibrium conditions.
A rinsing system with controlled pressure and temperature was used to remove electrolytes in a reproducible manner. Afterwards, the probe was dried and heated rapidly to a controlled temperature in a conforming oven in the hydrogen gauge. The gauge (Fig. 3) was also provided with appropriate voltages and currents so that the hydrogen pressure curve could
244
be recorded (Fig. 4). I,,,, the integral of that curve to the hydrogen pressure peak, HP, is electronically displayed, as is the hydrogen pressure peak, and h, the time in seconds from HP to HP/Z. Tile calibration pressure curve rises to a hydrogen pressure peak, HP, which is divided by 100 to give a “HP calibration factor”; future probe hydrogen exposures are divided by this factor in order to normalize them. The time in seconds, measured from switching on the probe to the hydrogen peak (HP) is called y. The measured integral of the pressure curve, from switching on to hydrogen peak, is I,, and the normalizing 1, factor is 1,/100. In the case of h (defined above), the normalizing factor is X (measured) 140. The use of these dividing factors normalizes the measurements made with different probes. Clean-up Hydrogen is completely removed from the probe before each test by electronically heating the probe while ionizing the hydrogen; this is referred to as clean-up or CU. The probe hydrogen pressure during outgassing can be monitored by using the hydrogen window as an ion collector. This permits precise and reproducible starting pressures to be obtained. After CU, the probe is cooled for at least one h,our prior to re-exposure. If the re-exposure is a second calibration, the measured I7 is closely reproducible. The HP increases by approximately 6% with each succeeding calibration, while h decreases at about the same rate. These effects are due to the decrease in the steel membrane thickness caused by the abrasion, accompanying surface activation. Probe reaction
kinetics
A knowledge of the heterogeneous equilibrium reactions which accompany probe operation is of value for the understanding of probe hydrogen reaction kinetics. Probe pressure differences, by themselves, do not give an accurate measurement of absorbed hydrogen in the system, because of hydrogen pumping (gettering~ and release. The differenti~ equation repre senting these reactions is: dP/dt = dH/dt - dG/dt
+ dlYJdt
where dP/dt is the observed pressure change with time, dH/dt the rate of hydrogen input from the shell, dHJdt the rate of hydrogen recirculation from collector plate and barium getter, and dG/dt is the rate of all pumpings, equals UP, where a is the pumping speed. The ion pumping speed is 1 probe volume per second at 6 mA ionizing electron current. The pumping by the barium getter used to produce the initial high vacuum is comparable; both reactions are reversible with increased
245
temperature; the release of hydrogen is proportional to the hydrogen absorbed and is exponentially dependent on the temperature. The Ba is located on the inner surface of the probe metal shell and its temperature will follow that of the shell, approaching 140 “C under oven conditions. The ionization plate will approach 140 “C! more slowl.y, due to its physical separation from the shell. Heat-transfer will be primarily due to radiation, since, at the low probe pressures, convection is small. The recirculation of hydrogen, by the plate and getter, produces an overall pressure decay curve that is not truly exponential. It is actually the difference of two exponentials; one due to a hydrogen permeation through the hydrogen window and the other due to recirculation effects. Where the hydrogen exit window is a combination of steel and palladium, as in earlier Pd-Ag probe construction, we observe that the log decrement of the decay rate is not equal for equal times but, on the contrary, that each succeeding time interval must be multiplied by a constant. At a temperature of 250 ‘C, in an atmosphere of heated air, this constant multiplier, for an initial 20 min increment, was found to be 1.2. The decay observed was 20% per increment and the relationship was maintained over 24 h. Since the major contribution to the probe pressure following HP has been found to be due to recirculation effects, integration of the hydrogen pressure curve to its peak, HP, has been found to be proportional to the hydrogen absorbed. Integration also provides a significant improvement over earlier methods where the HP alone was used. The measured value of HP is critically dependent on the exact oven temperature, the probe insertion into the oven, and the thermal environment of the oven. The integral to the peak (I,) is much less sensitive to these effects.
Experimental
equilibria
One can demonstrate equilibria of another kind by depositing hydrogen at different current densities and from different electrolytes (Fig. 5) and balancing this input with hydrogen desorption from the probe. When the solution was 0.01 M H,S04 and the current density 0.04 A dmw2, the hydrogen pressure plateau, representative of equilibrium conditions, at 5 X lo-’ Torr, was reached after 2$ h. When the hydrogen was deposited at 2 A dme2, a level of 3 x lop6Torr was reached after an additional hour of charging. Further increases in current density in this electrolyte created hydrogen bubbles which masked the surface and, in this case, the proportion of hydrogen absorbed by the probe was decreased. This effect is not observed, even at 8 A dmp2, when hydrogen “poisons”, such as cyanides, cover the surface and change the ratio between adsorbed hydrogen and recombined hydrogen [ 31. Agitation of the electrolyte also removes the blocking bubbles and increases sorption. Increases of temperature increase the proton diffusion rate. This removes adsorbed hydrogen ions from the surface and leaves a surface-active “vacancy” for readsorption of another hydrogen atom. Both factors increase the ratio of hydrogen sorbed to hydogen lost to molecule formation.
246 0T NON-EQUILIBRIUM
HEAVYBUBBLEFORMATION (SURFACE OCCLUDED)
TIME(HOURS) Fig.
5. Probe HP, h, 7, Zy, and I,
curve (ASD = A dm-‘).
Probe analog A simple analog to the probe hydrogen sorption and internal hydrogen pumping is obtained by visualizing the probe as a funnel, into which the hydrogen is introduced and from which it disappears at a rate proportional to pressure levels (Fig. 6). The probe pressure corresponds to the “depth” of the hydrogen in the funnel. At equilibrium, the pressure remains constant for each hydrogen input within the pumping capabilities of the probe. The equilibrium pressure may be increased or decreased as a function of the hydrogen input. The ion pumping by the collector may be turned off by turning off the probe electronically. The barium-getter pumping is reduced by lowering the hydrogen sorption temperautre. For example, the getter pumping is lowered more than 30-fold by making the test exposure at 20 “C rather than at 100 “C.
Measurement
of high hydrogen
inputs
The pumping may be used effectively for monitoring relatively high hydrogen input rates. This has been used for following the hydrogen sorption by steel during chromium plating at 50 ‘C, leaving the probe on. In this case, the equilibrium pressure level was maintained by two factors, i.e., internal hydrogen pumping and reverse hydrogen permeation through the steel window.
247 PROBE
FUNNEL ANALOG
PROBE OFF
ION Cp~m~CTOR
t!IN
."'+ BARIUM GETTER
H IN H' IN = H' OUTPROBE ON
* HYDROGEN PROTON
Fig. 6. Probe
funnel
'HYDROGEN ATOM
-HYDROGEN MOLECULE
analog.
These rates are proportional to internal hydrogen pressure. The latter effect, no doubt, increases the proportion of plated hydrogen that bubbles away, since it would tend to fill hydrogen-active sorption sites from the reverse side and thus lead to recombination reactions at the surface. The response time of the system is very rapid. With a steel window 0.4 mm thick, new equilibrium levels as a function of temperature and current density changes were established within minutes.
Steel window probes: plating In early techniques used for the control of low-embrittlement cadmium plating, a steel probe having a well-defined steel hydrogen window outlined by a special epoxy paint (Fig. 7), was surface-activated by abrasive blasting, plated to full specification thickness, rinsed, dried, turned on, and inserted into the small encircling oven of the hydrogen gauge. A hydrogen pressure curve us. time was obtained (Fig. 4). The important parameter empirically selected for control was h, the time between HP and HP/2, and this was
248
Fig. 7. Epoxy-masked steel hydrogen probe.
found to vary with the permeability of the applied plate of low-embrittlement cadmium plating. This plate permeability v&s of critical value for determining the time required to bake plated parts to remove hydrogen to safe levels. However, we encountered problems when we tried to apply the above techniques for the control of brightener levels in a copper pyrophosphate bath, since we had a relatively impermeable hydrogen barrier. Empirically, we were led to the “time-lag” method of determining the diffusion coefficient of the plate [4] . The data obtained gave a highly reproducible parameter, y, which varied with brightener concentration. As more brightener was added, the crystal structure of the electroplate became finer, and the hydrogen diffusion coefficient decreased as the plating became more impermeable to hydrogen. Many years have been spent in developing commercial X standards that correlate with hydrogen embrittlement thresholds in sustained load specimens of 4340 steel, heat-treated to 260 - 280 ksi, ultimate tensile strength. However, since the standard A deviation exceeds 100% for h greater than 120 s, early conversion to y standards will require either complete re-correlation studies to be made directly, or else some method of conversion between the two will have to be found. The gauge has proved useful in studying the difference between lowembrittlement and bright (high-embrittlement) cadmium-plating systems. In the former, most of the hydrogen deposited goes directly into the steel. Dissolving the plating in NH,NOa before inserting the probe into the oven, produces HP values which are almost identical with those measured without removal of the plate.
249
*+/ ‘:
STEEL-
4
;;L-d d,+ *
+ fi + I '+ ,;
lOOiO?At 90 I STEEL.CAOMhl
“I
.w I
J,’ Y ‘*/,
*,
/+, .: ,'*,',* ',, (+ ,/ '+ ,'.' I"Tr?Tlls / t + 101 STEELCAOMlhi ' '+,
I”
IYl”L
I
IO 1
5
’‘, /.!I ‘, ’ .‘!/ \ ’
",
/ ,
,
,,, , , / / / ' / ,',' IM :' oioSTEE~.CAOMIL , ,'/
100
100
L1
END OF BAKE
MID BAKE
AFTERPL, ATINGBEFOREBAKING
TIME(H6URS) 23
+ HYDROGEN PROTON
Fig. 8. Hydrogen
0 HYDROGEN ATOM
location
AFTERPLATINGBEFORE BAKING
m HYDROGEN MOLECULE
in low-embrittlement
cadmium-plated
MID BAKE
END OF BAKE
steel during baking.
FINE GRAINED CADMIUM STEEL5OOdTOTAt.' /, IOOO-1STEEL. CADMIUM
10.000 -
1000~1 STEEL CAOMIUM~"
5000
%
5
TIME(HOURS)23
+ HYOROGEN PROTON
Fig. 9. Hydrogen
0 HYOROGEN ATOM
location
in bright
(dense)
-HYDROGEN MOLECULE
cadmium-plated
steel during
baking.
In the case of bright (fine-grained) cadmium plating, the situation is quite different. Most of the hydrogen is still in the plate at the end of the electroplating time. If the plating is dissolved before placing the probe in the oven, the hydrogen input is reduced to only 1% of the value observed when the plating is left in place during the probe heating. This shows that, contrary to low-embrittlement practices, baking bright cadmium-plated parts increases, rather than decreases, the average hydrogen content of the steel (Figs. 8 and 9) [ 51. Confirmation is found in the work of Kudryavtsev etal. [6]. Steel window probes:
stress corrosion
For many years, low-embrittlement cadmium-plated steel probes have been used to measure hydrogen sorption from solutions that may, in service, contact similarly plated high-strength steel parts under stress. The use of the probe-heating step, which does not, in this case as in plating, simulate treatment of plated stress-rupture parts, has presented problems. Variations
250
exceeding 100: 1 in the time to failure of the stressed plated specimens themselves have added to the general confusion regarding correlation. Present techniques consist of exposing the probe for 24 h to the solution under test. This, since 1960, has provided an increased degree of protection to the airlines. This exposure time has proved satisfactory, contrary to tests made at one hour or 45 min; the latter cannot distinguish between safe, moderately safe, and embrittling fluids. There is still no acceptance of a single general test procedure. The difficulties associated with heating the probe after test exposure, are that, at that time, part of the hydrogen measured comes from the cadmium plate and part from the steel. Yet, in stress corrosion correlation studies, this kind of measurement has been compared with plated, exposed, stiess-rupture specimens that are sensitive only to hydrogen in the steel. These are not baked, like the probe, to drive hydrogen from the plate inwards. A solution to this problem is achieved by measuring probe pressure levels after test, prior to oven baking. This is done by measuring the probe pressure maximum between 30 and 60 s inclusive, immediately following rinsing at 50 “C and drying. This value can also be obtained by integrating the area under the hydrogen pressure curve between the two times. The test simulates maximum expected field-temperature conditions.
Palladium-silver
probes:
corrosion
and post plating measurements
Development of probes having a Pd-Ag, 75%/25% hydrogen window instead of steel, permits the gauge to be used for monitoring molecular hydrogen in liquids and gases. The competing reaction of hydrogen with oxygen to form water on the Pd-Ag surface is avoided by limiting the exposure to atmosphere to a maximum of 20 s. The heating is in an inert gas, such as argon, that has been selected to have a maximum of 10 ppm impurities, with 0 ppm H, or hydrocarbons. Hydrogen Pd-Ag window surface activation is both chemical and mechanical. The amount of hydrogen introduced during this step in the procedures is small, yet it provides effective competition for the filling of active sites on the Pd-Ag with hydrogen, instead of oxygen trapping prior to test exposure. The chemical is sodium hypochlorite and the polishing of the water-wet Pd-Ag surface is carried out with abrasive powders. Initial work with this probe was carried out by the United States Navy to see if it was possible to correlate probe molecular hydrogen sorption from aqueous corrosion reactions with tests such as salt spray, salt water, acetic acid tests, and EXFO (exfoliation susceptibility tests of different heat treatments of aluminum alloys) [7]. The results obtained by immersing the probe in the corroding solution for 15 min showed excellent correlation with the older, visual examination techniques. This technique is particularly suited to the Pd-Ag window because it uses the high hydrogen solubility in Pd-Ag at room temperature to act as a sponge, and the high diffusion rate above 250 “C to increase system response time and sensitivity.
253. TABLE
1
Hydrogen concentration Time (min.)\
in high-strength steel us. extraction
time and post-plate bake time
Condition
30 60
I
II
8.1 306
52
1.8
III
IV
0.1 2
34
1.6
V
VI
0.2
0.01
4.7
0.3
Additional applications have been made for measuring residual hydrogen levels in electroplated, high-strength steel after low-embrittlement cadmium plating. After the parts were processed by the following procedures, they were analyzed. The samples were enclosed in a 1500 ml chamber under a cover of 99.999% pure argon. The chamber was pressurized to 2 bar and returned to 1 bar 11 times, reducing the oxygen content by a factor of 103. The sample was then heated at 300 “C in an aluminium crucible (hydrogenfree metal). The Pd-Ag probe was then heated for 5 min at 250 “C and the equilibrium hydrogen pressure measured electronically. Allowances were made for hydrogen loss due to various probe reactions: I II III IV V VI
Acid activated + Cd plate + no bake. Acid activated + Cd plate + 3 h bake at 200 “C. Acid activated + Cd plate + 23 h bake at 200 “C. Sandblasted + Cd plate + no bake. Sandblasted + Cd plate + 3 h bake at 200 “C. Sandblasted + Cd plate + 23 h bake at 200 “C.
Table 1 lists the ppm of hydrogen found in each class of procedure after 30 and 60 min of heating at 300 “C. The amounts shown constitute only a part of the total adsorbed during the plating process. To obtain total hydrogen, one would have to remove the Cd barrier and heat the samples at higher temperatures. Distinct differences are observed between different sample classes and for extended bake times. The tests were made several months after the parts were plated. It is seen that the residual hydrogen in the acid-activated samples is nearly 10 times higher than that of the sandblasted ones. The results would have been higher for the acid samples had not their hydrogen effusion rate been reduced by darkening. It was observed that significant sample darkening occurred with the acid-treated samples. The effect of the plated surface barrier on hydrogen-loss measurements should not be overlooked. In the case of hydrogen analysis in a bright Znplated, high-strength steel hose clamp (Fig. lo), only 1 ppm of hydrogen was measured after 3 h bake at 200 “C. The sample was broken, exposing the unplated surfaces. On reheating, a hydrogen level of 1000 ppm was obtained in 10 min, as the hydrogen was then able to escape to the front from the unblocked samples.
Fig. 10. Zinc-plated
(embrittled)
high-strength
steel hose clamp.
Conclusion Past me~u~ements have indicated a number of different plating and corrosion parameters that may be studied with the hydrogen gauge and probes. More rapid hydrogen-content testing of samples has been made possible by induction heating, Using glass instead of metal for the Pd-Ag envelope improves the sensitivity so that hydrogen partial pressures of the order of low8 Ton can be measured.
References 1 2 3 4 5 6
F. J. Norton, J. Appl. Phys., 11 (1940) 262. J. 0. McCaldin, J. Appl. Phys., 27 (1956) 307. W. Beck, J. Electrochem Sot., 112 (1965) 53. R. M. Barrer, Trans. Faraday Sot., 36 (1940) 1235. S. C. Lawrence, ASTM 543,83, Hydrogen Detection Gage, presented V. N. Kudryavtsev, K. S. Pedan and A. T. Vagramyan; Electrodepos. (1972/73) 213. 7 P. Fischer, Report No. NADC-72251.VT, 1972.
June, 1972. Surface Treat.,
1