Effects of Electroplated Zinc-Nickel Alloy Coatings on the Fatigue Strength of AISI 4340 High-Strength Steel

Abstract

Recovered substrates have been extensively used in the aerospace field. Cadmium electroplating has been widely applied to
promote protective coatings in aeronautical components, resulting in excellent corrosion protection combined with a good performance
in cyclic loading. Ecological considerations allied to the increasing demands for corrosion resistance have resulted in the
search for possible alternatives. Zinc-nickel (Zn-Ni) alloys have received considerable interest recently, because these coatings
show advantages such as a good resistance to white and red rust, high plating rates, and acceptance in the market. In this
study, the effect of electroplated Zn-Ni coatings on AISI 4340 high-strength steel was analyzed for rotating bending fatigue
strength, corrosion, and adhesion resistance. The compressive residual stress field was measured by x-ray diffraction prior
to fatigue tests. Optical microscopy documented coating thickness, adhesion characteristics, and coverage extent for nearly
all substrates. Fractured fatigue specimens were investigated using scanning electron microscopy (SEM). Three different Zn-Ni
coating thicknesses were tested, and comparisons with the rotating bending fatigue data from electroplated Cd specimens were
performed. Experimental results differentiated the effects of the various coatings on the AISI 4340 steel behavior when submitted
to fatigue testing and the influence of coating thickness on the fatigue strength.

Full text

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EFFECTS OF ELECTROPLATED ZINC-NICKEL ALLOY
COATINGS ON THE FATIGUE STRENGTH OF AISI 4340
HIGH STRENGTH STEEL
H. J. C. Voorwald , I.M. Miguel and M.Y.P. Costa
Department of Materials and Technology, State University of São Paulo, Cep. 12516-410, BRAZIL.
ABSTRACT
It is well known that fatigue behaviour is an important parameter to be considered in mechanical components
subjected to constant and variable amplitude loadings. In combination with corrosion phenomenon, fatigue
effects were responsible for proximally 64% of fails that occur in metallic parts of aeronautical accidents in
the last 30 years. Recovered substrates have been extensively used in the aerospace field. Cadmium
electroplating has been widely applied to promote protective coatings in aeronautical components, resulting
in excellent corrosion protection combined with a good performance in cyclic loading. Ecological
considerations allied to the increasing demands for corrosion resistance, resulted in the search for possible
alternatives. Zinc-nickel alloys received considerable interest recently, since these coatings showed some
advantages such as a good resistance to white and red rust, high plating rates and acceptation in the market. In
this study the effects of zinc-nickel coatings electroplated on AISI 4340 high strength steel were analysed on
rotating bending and axial fatigue strength, corrosion and adhesion resistance. Compressive residual stress
field was measured by a X-ray tensometry prior to fatigue tests. Optical microscopy images showed coating
thicknesses, adhesion and the existence of an uniform coverage of nearly all substrates. The fractured fatigue
specimens were investigated using a scanning electron microscope. Three different zinc-nickel coating
thicknesses were tested and comparison with rotating bending fatigue data from specimens cadmium
electroplated and heat treated at 190°C for 3, 8 and 24 hours to avoid the diffusion of hydrogen in the
substrate, was performed. Experimental results showed effect of coatings on the AISI 4340 steel behaviour
when submitted to fatigue testing and the existence of coating thickness influence on the fatigue strength.
1 INTRODUCTION
In recent years, with higher worldwide demand for improved quality and corrosion resistance,
alternative coatings are required. Zinc alloys, particularly zinc-nickel, have been extensively
studied due to their technological application in protection against corrosion, as an alternative to
cadmium alloys [1-3]. These coatings are widely applied in automotive industry, normally with
thicknesses of 10 µm [4]. These alloys have also been considered for several applications such as
in electrocatalytic water electrolysis [5], as coatings for steel cord reinforcement of tires [6] and in
the electronics industry [7]. In the Brazilian aeronautical industry, despite of the excellent
properties as agent protective showed by Zn-Ni coatings, these alloys are still in study and in a
development stage. In this industrial segment, a good resistance to fatigue is required, which
result in another difficulty in the search of alternatives to cadmium as coatings in high strength
steel.
In case of aeronautical applications, protection against corrosion must be combined with fatigue
resistance.
The aim of this study was to analyze the effects on AISI 4340 steel, in the rotating bending
fatigue behavior, of zinc-nickel coating thickness electroplated on AISI 4340 steel and to compare
with the results obtained by cadmium electroplating. Fracture planes of the fatigue specimens were
examined using a scanning electron microscopy in order to identify the crack initiation points.
Optical metallographic was used to investigate the existence of an uniform coverage of nearly all
substrates.

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Studies of the resistance against corrosion were carried out using salt spray tests and the results
showed according to ASTM B117. The residual stresses promoted by machining and the
electroplating process were obtained and compared with base material.
Residual stresses were determined by X-ray diffraction method, using the Ray stress
equipment, whose characteristics are described in [8]. The accuracy of stress measurements was
∆σ = ±20 MPa. In order to obtain the stress distribution by depth, the layers of specimens were
removed by electrolytic polishing with a non-acid solution.
2 EXPERIMENTAL PROCEDURE
The base material used in this work was the AISI 4340 steel widely applied in aircraft components
where strength and toughness are fundamental design requirements. The chemical composition of
the steel is in accordance to required standards.
Rotating bending fatigue test specimens were machined from hot rolled quenched and tempered
bars. Fatigue tests specimens were quenched from (815 – 845) °C in oil (20 °C) and tempered in
the range (520 ± 5) °C for two hours. Mechanical properties of the steel after the treatment are:
hardness = (38 – 42) HRc, yield strength = 1118 MPa and ultimate tensile strength = 1210 MPa.
After final preparation, specimens were subjected to stress relieve heat treatment at 190 °C for four
hours with posterior cooling in still air, to reduce residual stresses induced by machining.
Specimens were machined according to specifications, polished with 600 grit papers, inspected
dimensionally and by magnetic particle inspection. Average superficial roughness in the reduced
section of the specimens was Ra ≈ 2.75 µm with standard deviation equal to 0,89 µm.
2.1 CADMIUM AND ZINC-NICKEL ALLOYS ELECTROPLATING.
The cadmium electroplating was carried out by rotative drum process, during 1 minute to 10
minutes to produce coating thicknesses in the range 8 µm to 13 µm. Parameters employed in this
process were: cadmium salt solution 20 – 30 [g/l], sodium cyanide solution 90 - 200 [g/L], sodium
hydroxide solution 10 – 20 [g/L], relations Cn/Cd equal 4:1 – 6:1, temperature room, continuous
current between 1 – 5 [A/dm2], voltage anode/cathode 1 – 4 [volts] and electrodeposition velocity
between 0,2 – 1,5 [µm/min]. Once finished the bath deposition, specimens were treated with
sodium cyanide solution for 15 seconds to 30 seconds, treated with stream water and dried with
hot air for 2 minutes to 4 minutes. The last step in the cadmium electrodeposition process was
dehydrogenation by heat treatment to avoid the diffusion of hydrogen in the substrate.
The zinc-nickel electroplating was carried out in accordance to the plating control parameters
shown in table 1. Prior to the deposition, pieces were submitted to the following sequence of
pretreatment: alkaline clean, acid pickling and rinsing with distilled water. After plating,
specimens were subjected to rinsing and drying and submitted to hydrogen embrittlement relief
treatment at 190 ºC for 24 hours.
2.2 ROTATING BENDING FATIGUE TESTS
Rotating bending fatigue tests were performed using a sinusoidal loading at frequency of 50 Hz
with stress ratio R equal to –1,0 in air and at room temperature. These tests consider as fatigue
strength complete fracture of the specimens or 107 load cycles. Nine groups of fatigue specimens
were prepared to obtain S-N curves for rotating bending fatigue tests.
2.3 RESIDUAL STRESS ANALYSIS
The residual stress analysis were conduced by portable X-ray apparatus with an air cooled double-
anode X-ray tube, provided with a stress measurement unit and focusing camera for phase
analysis. The characteristics of the equipment used in this analysis were showed by Monin et al

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[9] and the methodology applied during the tests is described. Specimens used in these tests have
the same specification adopted in the fatigue tests. Measurements were carried out with base
material and base material zinc-nickel alloy electroplated, at the interface substrate/coatings and
inside substrate.
2.4 FRACTURE SURFACE ANALYSIS AND METALLOGRAPHIC CHARACTERIZATION.
Fracture surface analysis was carried out on rotating bending fatigue test specimens by scanning
electron microscope, model Leo 435 Vpi. The metallographic analysis was carried out by optical
microscope model Neophot 21.
Images were used to identify the existence or not of an uniform coverage of nearly all
substrates and to investigate the better performance of the coatings with respect to their adherence
properties. Thicknesses of the coatings were measured and compared in order to verify their
relationship with fatigue behavior. The metallographic analysis was carried out on specimens
sectioned in the transversal orientation, inlaid work in acrylic resin, prepared using 80 at 1000
graduate waterproof papers and polished with 99,98% Alumina suspension.
2.5 RESULTS AND DISCUSSION
2.5.1 FATIGUE TESTS
Figure 1 shows SxN curves for rotating bending fatigue tests for the base material, hard chromium
electroplating, cadmium electroplating and zinc-nickel electroplating.
300
400
500
600
700
800
900
1000
1100
104105106107
BASE MATERIAL - AISI 4340.
BASE MAT. & ZI NC-NICKEL (7µm-12µm).
BASE MAT. & ZI NC-NICKEL (15µm-21µm).
BASE MAT. & ZI NC-NICKEL (2µm-6µm).
BASE MAT. & CA DMIUM (8µm-13µm), WITHOUT HYDR. EMBRITTL. RELIEF TREAT.
BASE MAT. & CA DMIUM (8µm-13µm), 3 HOURS/19 0 °c OF HYDR. EMBRITTL. RELI EF TREAT.
BASE MAT. & CA DMIUM (8µm-13µm), 8 HOURS/19 0 °c OF HYDR. EMBRITTL. RELI EF TREAT.
BASE MAT. & CA DMIUM (8µm-13µm), 24 HOURS/190°c OF HYDR. EMBRITTL. RELIEF TREAT.
BASE MAT. & CHROM IUM (160µm).
CYCLES (N)
STRESS (MPa)
Figure 1: Rotating Bending fatigue test. Based material, zinck-nickel alloys, cadmium and
chromium electroplated specimen.
Experimental data for base material in figure 1 will be used in this work as a reference to be

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compared with the fatigue strength of AISI 4340 steel electroplated with cadmium and zinc-nickel
alloys. It is possible to observe from figure 1 the significant reduction in the fatigue life of
specimens chromium electroplated.
The SxN curve for the specimens tested after cadmium electroplating, without post heat
treatment shows decrease in fatigue strength in comparison to the base material, for low and high
cycles.
To understand the effect of zinc-nickel electroplated coating on the SxN curves of AISI 4340
steel, figure 2 should be considered. From figure 2, which indicates residual stress at interface
substrate-coating and inside base metal it is possible to observe that compressive residual stresses
at interface decreases with the decrease in the coating thickness. Maximum compressive residual
stress at interface is obtained for base metal. Interesting is to observe the variation of residual
stress increasing the distance from the interface. After 0.1 mm, specimens zinc-nickel electroplated
coated with thicknesses in the range of 7 µm - 12 µm and 15 µm - 21 µm, showed tensile residual
stresses of 20 MPa and 60 MPa, respectively. For base material and zinc-nickel electroplated
coating in the range of 2 µm - 6 µm, table 1 shows that at 0.1 mm from the interface residual
stresses are – 260 MPa and 20 MPa, respectively.
0,0 0,1 0,2
-600
-500
-400
-300
-200
-100
0
100
BASE MATERIAL
2 µm - 6 µm - 39 HRc
7 µm - 12 µm - 39 HRc
15 µm - 21 µm - 39 HRc
RESIDUAL STRESS (MPa)
INTERFACE DISTANCE (mm)
Figure 2: Residual stress measurements tests.
At 0.2 mm from the interface, all conditions show tensile residual stresses, in the range 20 MPa
to 100 MPa. It is interesting to observe for zinc-nickel electroplated coating 15 µm – 21 µm, the
variation in the residual stresses from 500 MPa compressive at interface to 60 MPa tensile at 0.1
mm from this point. Comparison between residual stresses at 0.1 mm and 0.2 mm from the
interface indicate, exception made for the base metal, almost the same results, for zinc-nickel
electroplated coatings.
It is well known that delay in fatigue crack nucleation and propagation results from the
presence of compressive residual stress field which, consequently, increase fatigue strength.
Compressive residual stress at interface for thicknesses 2 µm - 6 µm, 7 µm - 12µm and 15 µm – 21
µm were 90 MPa, 210 MPa and 500 MPa, respectively. To understand the behavior of the SxN

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curves represented in figure 3, residual stresses at 0.1 mm from the interface coating-substrate
must be considered. The highest residual stresses was obtained for zinc-nickel electroplated
coating with thickness 15 µm – 21 µm, in comparison to 2 µm - 6 µm and 7 µm – 12 µm.
Figure 2 indicates residual stresses behavior with compressive values at interface and tensile at
0,1 mm from that point, for base material and the three zinc-nickel electroplated coating
thicknesses studied.
Figure 3: Optical microscopy. Thicknesses and homogeneity of zinc-nickel layers: (a) = 2 µm –
6 µm, (b) = 7 µm - 12 µm and (c) = 15 µm - 21 µm.
Figure 4: Scanning electronic microscopy. AISI 4340 zinc-nickel alloy coatings. Thicknesses
between 15 µm - 21 µm.
Figure 3 (a) shows an optical microscopy image of zinc-nickel electroplated coating, 2µm – 6
µm thick, with 1000 X magnification. It is possible to verify that the coating layer is uniform and

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there are no signs of detachment of coating from the substrate. Figures 3 (b) and 3 (c) represent
images from coating thicknesses 7 µm – 12 µm and 15 µm – 21 µm. Coating homogeneity and
strong adhesion with substrate are observed.
Figures 4 (a), (b) and (c) show represent fracture surfaces a rotating bending fatigue specimen
tested at 951 MPa, fractured after 10600 cycles. Electroplated zinc-nickel coating thickness was in
the range 15 µm – 21 µm. Figure 4 (a) represent the general fracture surface aspect with several
crack origin sites around specimen surface. Figure 4 (b), indicates fatigue crack nucleation at
interface coating-substrate. In figure 4 (c), fatigue crack nucleation at interface and propagation
inside substrate, is shown. Figures 4 (d), (e) and (f) are associated to a fatigue specimen tested at
571 MPa, with a number of cycles to fracture equal to N = 83600.
Figures 4 (e) and (f) indicate fatigue crack nucleation inside substrate, associated to the
variation in residual stresses from 500 MPa compressive at interface to 600 MPa tensile at 0.1 mm
from this region.
2.6 CONCLUSIONS
1. For low cycle fatigue, differences in fatigue strength specimens exposed to 190 °C for 2, 8
and24 hours were not observed. In high cycle fatigue some influence was detected, indicating
better results for lower treatment times.
2. Reduction in the AISI 4340 steel rotating bending fatigue strength resulted from zinc-nickel
electroplated coating in the thickness interval 15 µm - 21 µm. In the case 7 µm - 12 µm,
experimental data indicate almost the same behavior, with respect to the fatigue strength,
showed by AISI 4340 steel cadmium electroplated, 8 µm - 13 µm.
3 REFERENCES
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I. M. Miguel, C. E. A. Silva, M. P. Peres, H. J. C. Voorwald. Study of influence of zinc-nickel
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15-23.
2.
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G. F. Hsu, Zinc-nickel alloy plating: An alternative to cadmium. Plating and Surface Finishing.
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3.1 ACKNOWLEDGEMENTS
The authors are grateful for the support of this research by CAPES, ELEB/EMBRAER and
FAENQUIL/DEMAR.