The Use of Secondary Ion Mass Spectrometry (SIMS)

Paper No.

11290

2011

The use of Secondary Ion Mass Spectrometry (SIMS) in Quality Control of Electroplated and Baked High-Strength Steels

Elizabeta Kossoy Israel Air Force P.O. Box 02538 Israel

Noam Eliaz Tel-Aviv University Ramat Aviv Tel Aviv 69978 Israel

Gil Shemesh Israel Air Force P.O. Box 02538 Israel

ABSTRACT Hydrogen absorption during electroplating might result in hydrogen embrittlement (HE) of the substrate metal. Heat treatment ("baking") is commonly employed "in order to render the normally mobile hydrogen immobile". The objective of this work was to develop a sensitive analytical procedure using dynamic secondary ion mass spectrometry (SIMS) that would allow identification of improper baking during quality control. In all non-baked samples of AISI 4340 steel coated with cadmium, an increase in the hydrogen signal was found at the Cd/steel interface. In baked samples, either a peak was not observed at the interface, or it was found insignificant based on determination of the ratios between the hydrogen signals within the coating, interface and substrate. The results were reproduced after 16 months storage in a desiccator. The main effect of baking was found to be effusion of hydrogen from the interface and the substrate steel into the atmosphere. HE-related delayed failures may thus be explained in terms of a time-independent reservoir of hydrogen at the coating/substrate interface, rather than in terms of irreversible damage that occurred within the substrate during electroplating. These findings contradict some of the statements in textbooks and international standards. Keywords: Electroplating, baking, hydrogen embrittlement, hydrogen trapping, secondary ion mass spectrometry (SIMS), high-strength steels, cadmium coating, quality control, delayed failure.

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INTRODUCTION The entry of atomic hydrogen into high-strength steels, either during fabrication (e.g. during machining, welding, pickling, or electroplating) or during service (e.g. due to corrosion processes, or cathodic protection), might result in loss of ductility, cracking, or catastrophic delayed brittle failures at applied stresses well below the yield strength of the steel. This phenomenon has received several names, including hydrogen embrittlement (HE), and it often occurs in alloys that show no significant loss in ductility when measured by conventional tensile tests. Hence, several standard mechanical tests have been suggested to evaluate the susceptibility to either internal or environmental hydrogen.1–3 The susceptibility of steels to HE increases with tensile strength, and becomes severe in ultrahigh strength steels (UHSS), such as the old, but still common, AISI 4340 alloy steel. The time dependent subcritical cracking in such steels is typically intergranular (IG), along prior-austenite grain boundaries, at apparent threshold stress intensity (KTH) levels as low as 10 MPa m , and crack growth rates (da/dt) as high as 104 μm/s.4 Figure 1 illustrates typical microscopic characteristics of the brittle fracture surface of a steel in an aircraft landing gear that failed due to HE. The IG fracture is evident, together with hairlines and microvoids on the facets of the grains. These, as well as other characteristics of failures of high-strength steels due to HE, are reviewed elsewhere in detail.5

Figure 1: Fractography (scanning electron microscope image) of a steel part in aircraft landing gear that failed due to hydrogen embrittlement.

Electroplating of UHSS is a common approach to improve their corrosion resistance. However, this process is accompanied by hydrogen evolution, in particular in alloy systems with low Faradaic efficiency, such as hard chromium and – to less extent – cadmium. Some hydrogen is thus codeposited and might lead to HE, depending on its quantity, as well as on the chemistry, microstructure and hardness of the steel. Subsequent thermal treatment ("baking") is used to "render the normally mobile hydrogen immobile".1 It is usually required to apply the baking process within some time after the completion of plating, in order to prevent irreversible damage to the substrate steel. In order to evaluate the effectiveness of this treatment, mechanical testing of statistically representative quantities of the finished items must be conducted in the framework of quality control.1–3 In the most common test, specimens must meet or exceed 200 h using a sustained load test at 75% of the tensile or bend notched fracture strength.2 Cadmium (Cd) plating is used to minimize bi-metallic corrosion between high-strength fasteners and aluminum in the aerospace industry. Depending on the hardness of the steel, it is applied either by


electroplating or by physical vapor deposition. In the case of electroplating, baking at 190±14 °C should be done for 23 h for all parts heat treated to 160 ksi (36 HRC) and above.6 The baking should begin not later than 4 h after the completion of the plating process. Yet, hydrogen has lower diffusivity and higher solubility in Cd than in steel. Consequently, the Cd deposit becomes a hydrogen source during the initial stage of baking, thus possibly increasing the concentration of hydrogen in the steel substrate.7 In addition, the Cd layer acts as a diffusion barrier to outgassing of hydrogen during baking, and a significant concentration of dissolved hydrogen might remain in the steel, even after baking times as long as 100 h.8 Hence, the effectiveness of baking of parts made of AISI 4340 or other UHSS and coated with Cd has been questioned.7–9 After typical Cd plating and baking, 1–5 wppm residual dissolved hydrogen can be detected, which is sufficiently high to embrittle the AISI 4340 steel.4,10 In many practical cases of in-service failures, even when a failed item is suspected for HE, it is not easy to confirm whether all manufacturing processes were carried out properly. Then, in order to prevent failures of items from the same manufacturing batch, there is often no choice but to remove the entire batch from service. This solution is usually complicated and costly, in particular when considering critical aeronautical components. The objective of this work was thus to develop a sensitive analytical test procedure that would allow mapping of the residual hydrogen, either during quality control or during failure analysis of electroplated items. EXPERIMENTAL PROCEDURE

The experimental procedure was described elsewhere in detail,11 and will therefore be summarized here in brief. The Substrate, Coating and Sample Preparation

Rectangular samples were prepared from AISI 4340 steel plate hardened to 40–42 HRC. The thickness of the plate was approximately 4.5 mm. The samples were ground on a 120-grit SiC paper and cleaned ultrasonically for 1 min in 2-propanol. Following cleaning in an alkaline bath, Cd electroplating was applied for either 1.5 min (5 μm thick coatings) or 4 min (10–15 μm thick coatings), in accordance with QQ-P-416, type I, class 3.6 The first group of samples, designated herein as B, was immediately baked at 191 °C for 23 h. The second group of samples, designated herein as NB, was not baked. The Selected Analytical Technique: Secondary Ion Mass Spectrometry (SIMS)

In SIMS, a solid sample is bombarded with a focused primary ion beam (300 eV–30 keV). The primary ions are implanted into the sample down to tens of nm. A transfer of kinetic energy takes place between the surface atoms, and collision cascades are created (Figure 2a). Some collisions are oriented backwards toward the surface. If they have enough energy to overcome the surface barrier potential, atoms, molecules and molecular fragments are ejected from the sample surface. Most of these particles are neutrals, but a small fraction is ionized. These secondary ions, characteristic of the composition of the analyzed volume, are separated according to their mass-to-charge ratio and collected. There are two major types of SIMS instruments – static and dynamic. Preliminary experiments showed that the latter is more adequate for this specific work. SIMS has several important advantages: (1) All elements, from H to U, can be analyzed, (2) High mass resolution (up to 0.02 amu), (3) Very high sensitivity (as high as ppb), (4) Mapping of the lateral distribution of species is possible, (5) Depth profiling is possible. The main drawbacks of this technique are: (1) UHV-compatible and size-limited samples are required, (2) Very high sensitivity for surface morphology and surface contaminants, (3) Limited optical capabilities that make it difficult to find grains or local regions of interest for analysis, (4) Need for a qualified operator, (5) High cost.12


SIMS has already been applied to a limited extent in mechanistic investigations of HE and stress corrosion cracking, with some success, as well as to identify the traps for hydrogen isotopes in different alloys.13–20 However, none of these studies is applicable to quality control or failure analysis of real electroplated items because they: (1) Used deuterium in order to reduce the noise signal, (2) Used polished samples, (3) Quenched the samples at cryogenic temperatures or performed SIMS analysis quickly after hydrogen charging in order to prevent hydrogen desorption before SIMS analysis, (4) Used, in certain cases, specimens that are difficult to extract from in-service parts. The Instrument and the Analysis Procedure. A Cameca ims 4f dynamic SIMS instrument was used. A 18–25 nA, 5.5 keV Cs+ primary ion beam was rastered over a sputtering area that varied between 50 × 50 μm and 250 × 250 μm, while the diameter of the analyzed area at the center of the crater was 33 μm (Figure 2b). The secondary negative ions H–, Fe–, C–, O– and CdO– were monitored. The samples were biased at –4.5 kV. The samples were introduced into the SIMS chamber at least 12 h before the measurement in order to minimize the environment contribution to the hydrogen secondary yield. The base pressure before analysis was 2 × 10–10 Torr. The resulting crater depths were measured with the aid of a depth profilometer. Preliminary experiments revealed that analysis of fracture surfaces is not successful. Hence, it was decided to measure the hydrogen (and other ions) depth profiles by sputtering through the Cd electroplate into the substrate steel (Figure 2b). Such approach may be useful also in failure analysis, for example by acquiring depth profiles close and in parallel to the fracture surface (Figure 2c).

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Figure 2: (a) Schematics of the collision cascades in SIMS. (b) The mode of analysis through the Cd plating. (c) Possible sampling mode in the case of fractured samples.

RESULTS

Figure 3 provides a comparison of typical hydrogen depth profiles in B and in NB samples. The iron depth profile is also shown, in order to emphasize where the interface between the substrate and the coating is. All samples were stored in a desiccator for 56 days before the SIMS analysis. It is evident that the hydrogen signal was increased at the Cd/steel interface in the NB sample. The same behavior was observed in all NB samples. This local effect of hydrogen was supported by fractographic scanning electron microscope images,11 according to which IG fracture pattern prevailed along the circumference


of the sample, around the interface of the substrate with the coating. In order to determine to which extent the results depend on the region of analysis, SIMS depth profiles were recorded at three different positions in each sample. In all measurements, a peak in the signal of hydrogen was observed at the Cd/steel interface in NB samples (Figure 4c). In the case of B samples, either no hydrogen peak was observed at the Cd/Steel interface (Figure 4a), or a small hydrogen peak was observed at the interface. Fortunately, a simple complementary quantitative analysis allows differentiating between the − − B and NB samples. Let Hinterface be the strongest signal detected at the Cd/steel interface, Hcoating be − be the signal at the the weakest signal within the coating layer – closest to the interface, and Hsteel deepest point analyzed within the substrate steel. Then, it can be shown with statistical significance that − − − − − − Hinterface Hcoating and Hsteel Hcoating are higher while Hinterface Hsteel is lower in the NB samples.11

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Figure 3: Comparison between typical hydrogen depth profiles measured through the Cd plating, for both baked and non-baked samples.(1)

Thus, it may be concluded that baking resulted in a significant decrease in the concentration of hydrogen in the steel compared to its concentration in the Cd coating. In addition, while following baking the Cd/steel interface became less enriched in hydrogen compared to the Cd coating itself, the steel substrate became less enriched in hydrogen compared to the interface. This suggests that the main effect of baking was effusion of hydrogen from the substrate steel and the Cd/steel interface, through the coating, into the atmosphere. Two samples (one B and one NB), which were arbitrarily chosen and reanalyzed after 16 months storage in a desiccator, revealed the same effect. This remarkable finding has two important implications. First, it indicates that HE-related delayed failures of improperly baked electroplated items may be related to the time-independent accumulation of hydrogen at the coating/substrate interface, and not necessarily to irreversible damage that occurred in the substrate metal during fabrication. Depending on the concentration of hydrogen at the interface,

________________________________ (1)

Reprinted from E. Kossoy et al., The use of SIMS in quality control and failure analysis of electrodeposited items inspected for hydrogen effect, Corrosion Science, Vol. 50, pp. 1481-91, Copyright (2008), with permission from Elsevier.


blistering and delamination might occur, or the interface may serve as a reversible hydrogen trap that provides a reservoir of diffusible (mobile) hydrogen for the steel. In the latter case, in an event that energy is absorbed in the material which is sufficient to overcome the activation energy barrier for hydrogen detrapping from the interface, so that diffusion of hydrogen towards strong irreversible traps in the steel is enhanced, the base metal itself may become embrittled within its bulk. Second, it seems that there is no time constraint for the use of SIMS analyses in failure analyses of electroplated items suspected for hydrogen damage. 4

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Figure 4: Hydrogen (H–) depth profiles obtained from two baked (a–b) and one non-baked (c) samples. In each sample, the profile was acquired at three different regions.(1)

CONCLUSIONS

This paper demonstrated the ability to differentiate between baked and non-baked samples of AISI 4340 steel electroplated with Cd based on dynamic SIMS depth profiles through the coating. The following conclusions were drawn: 1. In all non-baked samples, an increase in the hydrogen signal was found at the Cd/steel interface. In baked samples, on the other hand, either a peak was not observed at the interface, or it could be ignored based on determination of the ratios between the hydrogen signals in the coating, interface and substrate. This finding contradicts a common thought that the excess hydrogen is dissolved in the steel. 2. The main effect of baking seems to be effusion of hydrogen from the interface and the substrate steel into the atmosphere. 3. The reproducible effect can be monitored even after a very long storage time of 16 months in a desiccator. 4. Hydrogen embrittlement related failures may be explained in terms of the time-independent reservoir of hydrogen at the coating/substrate interface, rather than in terms of irreversible damage that occurred within the substrate during electroplating.


ACKNOWLEDGEMENTS

The authors are grateful for the financial support of the Israel Ministry of Defense.

REFERENCES

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