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Work Order O99-9905
Cooperative Research Program



August 28, 1999


Notice:

This TECHNICAL REPORT has been reviewed and is approved for publication and public release. It may be reproduced for educational purposes provided acknowledgment of the copyright holder is provided.


Prepared by:
W. Craig Willan, P.E.
Omega Research Inc.

Checked by:
Kenneth E. Mason
Omega Research Inc.


Caution


The notched samples tested in this program were AISI 4340 low alloy steel, heat treated to 260-280 ksi, Kt factor of 3.1 nominal (standard embrittlement test sample configuration) Other steel alloys and heat treat ranges may respond differently to acid clean induced embrittlement. The reader and end user is solely responsible for determining the applicability of this data to their specific product and application.


* removed for proprietary reasons.

Introduction


The data contained in this report has been generated under a Cooperative Research Program in conjunction with * and Omega Research of Southlake Texas. The research program was initiated due to ongoing confusion within the aircraft-aerospace community on the effects of acid clean-pickling on high strength steels and potential hydrogen embrittling effects on the parts subjected to the acid clean. All test solutions were prepared by * All embrittlement testing was performed at the laboratories of Omega Research, Southlake, Texas with test samples provided by Green Specialty Service, Fort Worth, Texas. The test sample matrix was jointly developed by *, and Craig Willan of Omega Research.

Test Approach


The most common cleaning-pickling acid, HCl, was utilized in 8 variations to evaluate its effect on hydrogen embrittlement of high strength steels. Test variations were:

Non-Inhibited HCl acid @ 40 and 60% Concentration (diluted from technical grade of 37%) each concentration non-aged and aged with iron. (4 acid variations)

Inhibited HCl acid @ 40 & 60% concentration (diluted from technical grade of 37%) each concentration non-aged and aged with iron. (4 acid variations)


The embrittlement testing method was the standard sustained load, 200 hours at 75% of notched ultimate tensile strength, with time-to-failure and applied load being the two controlled test variables. Each of the 8 acid variations were evaluated at various acid immersion times to produce an algorithm matrix suitable for use in determining permissible acid cleaning guidelines. Included in the 8 variation acid tests were HCl acids representing "aged" or production type acid cleaning baths. These acid solutions were prepared using 200 ppm of FeCl2 dissolved in the acid to simulate real world acid cleaning solutions with normal iron pickup over time.

Prior guidelines concerning acid cleaning or pickling tend towards limiting acid exposures to "..momentary..." without specific acid concentration limits, or restrictions towards inhibited vs. uninhibited acid types. In addition, the effects of iron build up over time in the acid solution have never been evaluated to the knowledge of * or Omega Research. All testing "momentary dips" was performed on standard stress rupture test frames, with calibrated dead weight-reacted load application of stress. Test frame calibration was per ASTM E-8, with maximum bending strain held to +/- 2% per ASTM E292 (max allowed 10%) Laboratory temperatures were held constant at 75 deg. F.

Discussion


Initial data points generated brought to light an interesting embrittlement phenomenon, that is room temperature bake or relief is observed on acid cleaned - bare steel samples. An appreciable room temperature diffusion of hydrogen out of the test samples is occurring during the transfer from acid exposure/rinse to actual testing. Since this test program involves no subsequent metallic or sealing coating applied after exposure, a free-surface diffusion gradient is present for hydrogen escape. The diffusion thermodynamics at 75 deg. F room temperature are sufficient for hydrogen migration either out of the samples, or into the samples to concentrations below which embrittling interactions do not occur (embrittlement threshold). It was not desired to add another test variable to this program, i.e. a metallic or other impermeable coating to prevent this hydrogen migration, as metallic coatings would generate additional hydrogen and add another variable difficult to quantify. From the initial stages of the test program, it was found that transfer dwell times of 2 hours or more allowed room temperature relief from embrittlement to occur. It must be emphasized that real world plating shop practices would apply an impermeable coating (at room temperature) quickly after acid clean. Entrapment of hydrogen from the acid cleaning would occur, along with any hydrogen generated during the actual plating process. Baking was not part of this test program, as it is well known that quick elevated temperature baking of bare steel samples would result in rapid alleviation of hydrogen effects.

As such, it became necessary to develop a test scenario that could realistically attempt to evaluate only acid cleaning effects. After exploratory testing, it was decided that utilizing a five minute dwell transfer time from acid/clean rinse to test loading would be appropriate. This 5 minute time began upon removal from acid/fast water rinse, to the time of actual application of test loads. The five minute dwell was deemed realistic in that real world plating shops in this time delay range. Dwell transfer timing was accomplished via digital devices, calibrated to N.I.S.T. WWV 10MHz broadcasts, and was held to a time tolerance of +/- 5 sec. Immersion timing was accomplished with a tolerance of +/- 2 sec.

It is generally believed that with bare surface, any embrittlement relief is effected via a free surface-to-atmosphere transfer, i.e. surface outgassing proceeds with lower energy requirements. This room temperature bake or relief effect has also been observed and addressed in the technical requirements of DOD-P-16232, Phosphate Coatings.

Acid inhibitors tend to be organic compounds, viscous in nature, and generally not liked by many metal finishing companies. This can be due to adhesion problems sometimes encountered with inhibited acid baths, on-going viscosity increases during time or aging, and also the fact that an inhibited acid bath tends to not clean and activate a ferrous surface as quickly as straight acid bath formulations. However, the result of this research program again verifies the ability of inhibitors to prevent embrittlement on high strength steels from HCl cleaning-pickling solutions.

Summary & Conclusions


  1. Acid inhibitors such as Rodine 213 appear to provide good protection against hydrogen absorption during acid cleaning, the mechanism being hydrogen pickup by the organic inhibitor compounds.


  2. An acid bath aged with iron appears to provide similar protection against hydrogen absorption, most probably through hydrogen interaction with iron oxides. Additional test work should be accomplished prior to allowance of this chemical phenomenon in lieu of actual proven inhibitors.


  3. Non-inhibited acid immersion times of 10 seconds maximum did not produce failures on high strength 4340 low alloy steel heat treated to 260-280 ksi. Martensitic steels of lower hardness/tensile strength would normally respond with even less sensitivity to embrittlement than the test samples used for this program.


  4. The effect of acid bath inhibitor aging was not explored in this program, and its effect of diminished protection against embrittlement over time is unknown.


  5. This test program was intended to investigate only the effects of acid cleaning on embrittlement. As such it was found prudent to eliminate any additional embrittling effect of top coatings or plating. The application of subsequent platings or coatings on top of the acid cleaned surface most probably will generate additional hydrogen pickup; thus prudent baking embrittlement relief is needed. Non-porous platings or coating may not allow sufficient hydrogen embrittlement relief, with the possibility of failures occurring where unexpected.




Test Matrix

Non-Inhibited Acid


SAMPLE I.D. AGED / NON AGED ACID CONCEN. IMMERSION TIME FAILURE TIME
AA non-aged 60% 120 sec. 5 sec.
BB non-aged 60% 120 sec. 98 min
GGG aged 60% 120 sec. nf
HHH aged 60% 120 sec. nf
Y non-aged 40% 120 sec. 6sec
Z non-aged 40% 120 sec. 5 sec
EEE aged 40% 120 sec. nf
FFF aged 40% 120 sec. nf
GGG-A aged 60% 120 sec. (1min transfer) nf
HHH-A aged 60% 120 sec. (1min transfer) nf


Conclusions: An aged acid clean bath acts as an inhibitor, appearing to prevent embrittlement. Non-aged acids at both 40 and 60% concentrations will produce failures when immersed for 2 min. (120 sec.). Additional tests (GGG-a and HHH-a) to confirm long immersion times times in aged baths as acceptable were performed with a quicker 1 minute transfer dwell time to make sure any embrittling effects were not masked by a 5 minute transfer dwell. No failures occured here either.

SAMPLE I.D. AGED / NON AGED ACID CONCEN. IMMERSION TIME FAILURE TIME
S non-aged 60% 60 sec. 43 min.
T non-aged 60% 60 sec. 49 min.
YY aged 60% 60 sec. nf
ZZ aged 60% 60 sec. nf
Q non-aged 40% 60 sec. 52 min.
R non-aged 40% 60 sec. 50 min.
WW aged 40% 60 sec. nf
XXX aged 40% 60 sec. nf


Conclusions: An aged acid bath acts as an inhibitor, appearing to prevent embrittlement with immersion times of 60 seconds. Non-aged, non-inhibited acids at both 40 and 60% concentrations produced failures.

SAMPLE I.D. AGED / NON-AGED ACID CONCEN. IMNMERSION TIME FAILURE TIME
1 non-aged 60% 30 sec. 170 min
2 non-aged 60% 30 sec. 133 min
3 aged 60% 30 sec. nf
4 aged 60% 30 sec. nf
5 non-aged 40% 30 sec. 28.6 hr
6 non-aged 40% 30 sec. 35.1 hr
7 aged 40% 30 sec. nf
8 aged 40% 30 sec. nf


Conclusions: An aged acid clean bath acts as an inhibitor, appearing to prevent embrittlement with immersion times of 30 seconds. However, non-aged acids at both 40 and 60% concentrations will produce failures.

SAMPLE I.D. AGED / NON-AGED ACID CONCEN. IMMERSION TIME FAILURE TIME
9 non-aged 60% 20 sec. nf
10 non-aged 60% 20 sec. 143.2 hr.
11 aged 60% 20 sec. nf
12 aged 60% 20 sec. nf
13 non-aged 40% 20 sec. nf
14 non-aged 40% 20 sec. nf
15 aged 40% 20 sec. nf
16 aged 40% 20 sec. nf


Conclusions: Immersion times of 20 seconds resulted in failures in one non-aged 60% concentration sample. No failures were recorded with aged solutions, nor were any failures recorded with 40% acid concentrations.

SAMPLE I.D. AGED / NON-AGED ACID CONCEN. IMMERSION TIME FAILURE TIME
K non-aged 60% 10 sec. nf
L non-aged 60% 10 sec. nf
QQ aged 60% 10 sec. nf
RR aged 60% 10 sec. nf
I non-aged 40% 10 sec. nf
J non-aged 40% 10 sec. nf
OO aged 40% 10 sec. nf
PP aged 40% 10 sec. nf


Conclusions: Immersion times of 10 seconds are not sufficient to induce embrittlement at 60% concentrations, non-inhibited acid, aged or non-aged bath.



SAMPLE I.D. AGED / NON-AGED ACID CONCEN. IMMERSION TIME FAILURE TIME
EE non-aged 60% 120 sec. nf
FF non-aged 60% 120 sec. nf
KKK aged 60% 120 sec. nf
LLL aged 60% 120 sec. nf
CC non-aged 40% 120 sec. nf
DD non-aged 40% 120 sec. nf
III aged 40% 120 sec. nf
JJJ aged 40% 120 sec. nf


Conclusions: Organic type inhibitor, such as Rodine 213, appears to provide effective protection against hydrogen absorption. Additional tests of inhibited acids at immersion times of less than 2 min (120 sec.) were not performed as no failures were recorded at the longer immersion times.



 
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NOTE:
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