ELECTRODEPOSITION ON SUPERALLOY SUBSTRATES: A REVIEW · source of Pt with sodium citrate and ammonium chloride as supporting electrolyte and complexing agent, the current e±ciency

ELECTRODEPOSITION ON SUPERALLOYSUBSTRATES: A REVIEW

M. H. ALLAHYARZADEH, M. ALIOFKHAZRAEI*

and A. SABOUR ROUHAGHDAMDepartment of Materials Engineering, Faculty of Engineering,

Tarbiat Modares University, P. O. Box: 14115-143, Tehran, Iran*[email protected], [email protected]

Received 20 August 2015Revised 30 October 2015

Accepted 23 December 2015Published 29 February 2016

The present paper reviews various types of coatings, including platinum, platinum alloys, palladium,ruthenium, iridium, nickel, nickel alloys and composite coatings, on superalloy substrates using elec-trodepositionmethod. Attempts were carried out to represent an overall view of plating conditions andelectrolyte and highlight the importance of the layer regarding to the performance of high-temperaturecoatings applied on superalloys, which is extensively used on gas-turbine components.

Keywords: Electrodeposition; superalloy substrate; coating; high temperature.

1. Introduction

Nickel-based superalloys, which are extensively used

in gas-turbine engine components, require protection

against high-temperature oxidation during operation.

In order to protect nickel-based superalloys from

oxidation and hot corrosion, di®usion aluminide

coatings are widely used.1,2 Being used since 1950s,

aluminide coatings based on the �-NiAl phase

deposited by the chemical vapor deposition (CVD)

method do not meet the requirement of the oxidation

resistance at high temperature. In this regard, Pt-

modi¯ed aluminide coatings can protect blades from

oxidative gases even at temperatures above 1400�C.3

It is reported that addition of Pt and some of the

platinum group metals (PGMs) in the aluminide

coatings causes formation of some phases (such as the

PtAl2 phase in Pt-modi¯ed coatings) in the coating

microstructure.4,5 The low ductility of these phases

and large di®erence of thermal expansion coe±cients

between the substrate and PGMs causes coating

degradation under the cyclic thermal stress.3,6–9

These coatings were prepared by electrodeposition of

a thin (few-micron-thick) layer of platinum or PGM

and subsequent aluminizing by pack cementation

or CVD. Therefore, the electrodeposition of such

metals and alloys is our interest because of their im-

portant role and mission to improve the temperature

resistant coatings, considering their high melting

temperature, excellent chemical stability and low

oxygen di®usivity.10,11 Recent investigations carried

out on high-temperature coatings, especially di®usion

and overlay coatings, demonstrated that electro-

chemical deposition of thin layer (3–7�m) of a spe-

ci¯c coating would enhance the service life of high-

temperature coatings at elevated temperatures. For

example, it was reported that electrodeposition of

*Corresponding author.

Surface Review and Letters, Vol. 23, No. 3 (2016) 1630001 (14 pages)°c World Scienti¯c Publishing CompanyDOI: 10.1142/S0218625X1630001X

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http://dx.doi.org/10.1142/S0218625X1630001X

thin (few-micron-thick) layer of platinum enhances

the aluminide di®usion coatings. This process

improves the service life of the aluminide coatings up

to three times.12 In the case of aluminide coatings, the

aluminum reacts with platinum to form platinum

aluminide, ¯nally leading to the formation of �-NiAl

phase. It was also reported that the formation of

PtAl2 phase at the interface of coating/superalloy

acts as a di®usion barrier to hinder the di®usion of Al

from the coating into the substrate and di®usion of

alloying elements from the substrate to the coating.

Hence, the desired properties of coating and super-

alloy preserve more than before during high-temper-

ature services. In this regard, the electrodeposition of

a thin layer of di®erent metallic, alloy or layered

structure coatings is of interest because of the im-

provement and enhancement of high-temperature

coatings. These electrodeposited layers could enhance

the oxidation and hot corrosion resistance of high-

temperature coatings by increasing the adhesion of

�-Al2O3 oxide layer to the coating, formation and

growth control of thermally grown oxide (TGO),13

etc. Therefore, the technology of electrodeposition on

superalloy substrates and, consequently, the appli-

cation of high-temperature coatings are very impor-

tant. Furthermore, because of their intrinsic

characteristics and alloying elements, superalloys are

loaded by surface oxides and intermetallic phases, i.e.

� 0-phase in � matrix. These phases introduce some

problem and di±culties to the electrodeposition pro-

cesses. Since the knowledge about the electrodeposi-

tion on the superalloy substrates is very limited, the

present study was conducted to review the electro-

deposition of platinum and its alloys. Besides, some

important metallic and composite thin layers were

electrochemically deposited on superalloys. Attempts

were performed to represent the plating conditions

and electrolyte and the importance of the electro-

deposited layers regarding the performance of high-

temperature coatings applied on superalloys.

2. Surface Preparation of Superalloys forElectroplating

Superalloys, like stainless steels, are readily passiv-

ated by the formation of surface oxide ¯lms. Such

¯lms indicate particular problems and di±culties

during electrodeposition process, which are regarding

the adhesion of plated ¯lms. Thus, special cleaning

and processing are required to eliminate these oxides

and surface ¯lms prior to plating.14 There are di®er-

ent types of surface activation and preparation before

electrodeposition process. ASTM B343 (Standard

practice for preparation of nickel for electroplating

with nickel15), ASTM B558 (Standard practice for

preparation of nickel Alloys for Electroplating16) and

ASTM B254 (Standard practice for preparation and

electroplating on stainless steel17) are the examples of

various standards and procedures carried out before

electrodeposition. Authors use various methods to

activate and prepare superalloys before deposition,

such as using anodic etching in a speci¯c activation

solution containing 55–80 vol.% of 95–98% sulfuric

acid (H2SO4) and 1–10 vol.% of 52% hydro°uoric acid

(HF). This type of activation, where the samples are

employed as the anode and using an inert cathode

such as lead, is suitable for activating stainless steels-,

nickel-, cobalt- and iron-based superalloys. It is also

reported that an intermediate rinsing with water

before electrodeposition tends to decrease adhesion of

the metal deposited to the substrate. Therefore, the

substrate can be preferably electrodeposited directly

following the activation treatment. The preferred

aqueous activating bath and conditions for this pro-

cess are 65 vol.% of 95–98% H2SO4 and 5 vol.% of HF

at 30Adm�2 and 20�C for 1min.14 Some researchers

prepared their samples through mechanical polishing

using emery paper down to 2000 grit. To do so, they

¯rst degrease the samples with acetone in an ultra-

sonic bath for 10min and then immerse them in a

30wt.% hydrochloric acid solution at 30� for 5min.6

Another surface preparation for nickel-based super-

alloys was carried out by mechanical polishing with

SiC paper up to 600 grit and degreasing with acetone

for 10min in an ultrasonic bath and then thoroughly

cleaning the surfaces with NaOH solution (1molL�)for 3min and hydrochloric acid (HCl) solution

(1molL�) for 30 s in an ultrasonic bath.8

According to Zagula-Yavorska and Sieniawski,

surface degreasing, surface etching, rinsing in cold

water and surface activation are the consecutive steps

for surface preparation.3 In their work, degreasing

was carried out in gluconate electrolyte at a temper-

ature of 50�C, where the electrolyte containing

180–240 gl� sodium gluconate (C6H11NaO7) and 18–

240 gl� sodium hydroxide (NaOH). Next, rinsing step

was conducted in hot water in order to clean the

samples' surface from C6H11NaO7 and NaOH.

M. H. Allahyarzadeh, M. Aliofkhazraei & A. S. Rouhaghdam

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Finally, surface activation was carried out using

etching in a mixture of 90 gl� HF þ 530 gl� HNO3.3

In another work, Zagula-Yavorska et al.18 prepared

their substrates using degreasing, electrolytic etching

and surface activation. The degreasing was performed

using 60 gl� NaOH and 30 gl� sodium carbonate

(Na2CO3) at 60�C. The electrolytic etching was per-

formed in a bath consisting of 100 gcm�3 H2SO4 and

20 gcm�3 sodium chloride (NaCl) at room tempera-

ture. Besides, surface activation was done in a bath

comprising 20 gcm�3 nickel dichloride (NiCl2) and

120 gcm�3 HCl at room temperature. Saremi and

Bahraini,19 ¯rstly, treated their samples anodically in

concentrated H2SO4 (65–70%) at room temperature

and at anodic current density of 20Acm�2 for 1min

to provide the required roughness (based on ASTM

B558). Table 1 illustrates the di®erent surface pre-

parations in brief.

3. Platinum Electrodeposition

Electrodeposition of platinum is practicable using

chloride, ammine, sulfate–nitrite and hydroxyl com-

plexes. Since platinum promotes hydrogen evolution

reaction (HER) considering the good catalytic activ-

ity of freshly deposited platinum and, on the other

hand, hydrogen in its turn has very low overpotential

for HER on Pt, generally, application of high current

density during Pt plating is impractical.20

There are many types of plating bath for Pt in the

literature. The oldest one dates back to a French

patent granted to Pilet in 1883, where platinum exists

in the bath solution as chloroplatinic acid,

H2PtCl6 � � � 6H2O, commonly referred to as \platinum

chloride".21 This type of plating bath was reported

practical for 2.5–3.5Adm�2 current densities and

with 15–20% cathodic current density.20 Literature

review also indicates that using (NH4)2PtCl6 as the

source of Pt with sodium citrate and ammonium

chloride as supporting electrolyte and complexing

agent, the current e±ciency increases up to 70% with

low applied current density.20 Crack-free crystalline

layers of platinum can be deposited from these baths

up to a thickness of 20�m at a temperature range of

45–90�C.Regarding the electrodeposition of platinum on

superalloy substrates, there are widely-used common

Pt plating electrolytes, as an electrolyte system based

on a platinum salt named dinitrodiammineplatinum

known as P-salt, Pt(NO2)2(NH3)2 (see Refs. 22–24)

and [(NH3)4 Pt(HPO4)], known as Q-salt,23,25 which

are reported in several classical papers and patents.

Table 1. Surface preparation of superalloys and activation phenomena.

Degreasing Supplementary activation Conditions Substrate Ref.

Degreasing in electrolytecontaining 180–240 gl� sodiumgluconate and 18–240 gl�

sodium hydroxide at 50�C.

Activation in the mixture of90 gl� HF þ 530 gl�

HNO3.

— Inconel-713 LC 3

Degreasing with acetone in anultrasonic bath for 10min.

Activation in 30wt.% HCL. At 30�C for 5min. TMS-75 6

Degreasing with acetone for10min in an ultrasonic bath.

Activation ¯rstly in 1MNaOH and then 1M HClsolution.

1min for NaOH and 30 sfor HCl.

TMS-82t 8

— Activation in a mixture of65 vol.% of 95–98%H2SO4 and 5 vol.% ofHF.

30Adm�2 and 20�C for1min.

General (US patent) 14

Degreasing with sodiumhydroxide 60 gl� and sodiumcarbonate 30 gl�.

Electrolytic etching with H2SO4

100 gcm�3 and sodium chloride

20 gcm�3 at 25�C.

Activation in nickel chloride

20 gcm�3 and HCl

120 gcm�3 at 25�C.

Degreasing at 60�C.Etching and activation

at 25�C.

Inconel-713 LC & andCMSX-4

18

— Activation in concentratedH2SO4 (65–70%).

at 20Acm�2 for 1min at25�C.

Inconel-738 19

Electrodeposition on Superalloy Substrates: A Review

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These systems allow a maximum current density of

5Adm�2. Addition of excess of sodium or potassium

nitrite to chloroplatinic acid (IV) results in reduction

of Pt(IV) to Pt(II) state to form the square planar

complex K2Pt(NO2)4 with evolution of nitric oxides.

Hence, addition of stoichiometric amounts of ammo-

nia leads to the precipitation of crystalline cis-Pt-

(NH3)2(NO2)2 (or Pt-P-salt). Reports demonstrated

that these systems give good deposits up to 7.5�m,

whereas current e±ciency varies from 10% to 40%

depending on supporting electrolytes (ammonium

nitrate, sodium nitrate and ammonia) and presence of

high working temperature varying from 70�C to

90�C. Rashidghamat et al.26 utilized P-salt electro-

lyte at 91�C with a solution pH of 10–10.5 and Pt

concentration greater than 12 gl� in their work. They

plated hemispherical centers of Pt mostly in the form

of °at deposits on Rene-80 superalloy. Their results

demonstrated that deposition rate of platinum layer

linearly increases with current density in the range of

0.3–0.5Adm�2; however, by further increasing the

current density beyond the critical amount (over

0.5Adm�2Þ, a sharp loss in the plating e±ciency was

observed. Accordingly, the average coating thickness

increased from 3�m to 5.5�m and then decreased

over the critical current density to 3.6�m. They have

also represented that the Pt size decreased with cur-

rent density, while it increased with duration of

electrodeposition. Zagula-Yavorska et al.3,18 utilized

this type of bath for Pt plating on Inconel-713LC and

CMSX-4. The appropriate current density in their

work was reported as 0.1Adm�2 in an electrolyte with

15 gl� tetraamine-platinum. Coating surfaces after

platinum electrodeposition with thicknesses of 3 and

7�m had a closely spaced \cauli°ower" appearance.

They used titanium as an anode for electrodeposition.

In elsewhere, a similar electrodeposition bath based on

[Pt(NO2)2(SO4)]2 was stated by Hopkin andWilson in

literature.22 Although this bath can be used for plating

on many metals such as Ti, Cu even at room temper-

ature, it is not very common for plating on superalloys.

Some researchers deposited platinum from commercial

bath.8,27 In comparison, signi¯cant developments in

platinum electrodeposition have been recently carried

out by Johnson Matthey, Ltd., and other scien-

tists.27–30 JohnsonMatthey, Ltd., patented a new bath

formulation based on platinum–tetraammine complex

(Tetraammineplatinum (II) dihydroxide: [Pt(NH3)4]-

(OH)2Þ in a phosphate bu®er with higher cathode

current e±ciency, which is as useful as commercial

electrodeposition bath, where high rates and thickness

are required. These types of bath were found suitable

for platinum plating on superalloy substrates. It is

noted that high temperature is needed for the reduc-

tion of platinum, as it is essential to drive the slow

ligand displacement reaction to a reasonable rate. The

mechanism was shown to be stepwise replacement of

ammonia ligands by water molecules. Deep funda-

mental studies of themechanism and applied aspects of

this new bath were conducted in the literature.31–35

Koslov36 patented the bath formulations to electroless

autocatalytic plating of platinum onto a superalloy

substrate such as Co-superalloy and Inconel.

It is reported37 that for high-temperature appli-

cations, the Pt presence in the aluminide coatings

system (Pt-modi¯ed aluminide coatings) reduces void

precipitation of secondary harmful phases at the in-

terface of metal/oxide. These coatings are typically

composed of dual phase structures of �-NiAl þ PtAl2having at least 45 vol.% of PtAl2.

38 Figure 1 repre-

sents the cross-section microstructure of the post-

heat-treated NiAl coating. Three distinct zones along

the cross-section of the coating were characterized.

The outer zone consists of an aluminum-rich NiAl

matrix dispersed with substrate element precipitates.

This zone is identi¯ed as \precipitated high alumi-

num NiAl" in Fig. 1.

Fig. 1. The cross-section back-scatter micrograph of theNiAl coating on superalloy substrate. Source: Reprintedwith permission from Shirvani et al.38 Copyright: Elsevier(2012).


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It has been also con¯rmed that Al uptake is fa-

vored by the presence of Pt at the outermost layers of

the coating using a low activity process. Furthermore,

the Pt content at the outer surface of the coating

decreases sharply toward the substrate when longer

di®usion treatments are conducted prior to alumini-

zation. Other studies39–41 show that platinum accel-

erates aluminum di®usion, then reduces the vacancies

°ux from the metal to the surface and inhibits va-

cancy coalescence and internal void formation. Pt is

known to enhance scale adhesion by reducing the

segregation of harmful elements such as S to the

scale/coating interface and/or by reducing void for-

mation at the interface. Researchers42 also reported

that platinum decreases the di®usive °ux of alloying

elements to the coating, promotes a selective alumina

scale formation and accelerates healing after spall-

ation. Pt slightly accelerates the alumina scale

growth and delays transformation of metastable

alumina oxides to �-Al2O3. The addition of platinum

to an aluminide coating improves the stability of

�-NiAl phase and delays the transformation of �-NiAl

to the � 0-Ni3Al phase. Table 2 presents the Pt plating

bath and conditions brie°y.

4. Platinum Alloy Electrodeposition

To improve the protective performances of the

Pt-modi¯ed aluminide coatings, much attention has

been paid to Pt-based binary alloy coatings, which

are reported throughout the literature.43,44

Pt–Ir alloy coatings from amidosulfuric acid solu-

tions (HOSO2NH2Þ were electrodeposited on nickel-

based single crystal superalloy TMS-75 using electro-

deposition.6,9 Researchers investigated the e®ects of

plating electrolyte temperature, current density and

mole concentration ratios of [Ir3þ]/[Ir3þ]þ[PtCl2�6 ] on

the deposition rate, composition and crystallographic

structures of platinum–iridium alloy coatings. The

composition of chloride bath used in this work is

iridium chloride: IrCl3 � � � 4H3O (0–40.0 gl�), chlor-

oplatinic acid hexahydrate: H2PtCl6 � � � 6H2O

(0–55.5 gl�), amidosulfuric acid: HOSO2NH2 (38.0–

48.0 gl�), and sodium nitrate: NaNO3 (40.0–50.0 gl�).The solution pH was also adjusted to 2 using 2N HCl

or 1N NaOH. They showed that when the current

density increases from 0.5Adm�2 to 4Adm�2, the Ir

content in Pt–Ir alloy coatings increases from 23.6 at.%

to 30.5 at.%. Their results also con¯rmed that the de-

position rate of the coatings linearly increases as cur-

rent density rises (Fig. 2). These results demonstrated

that with increasing electrolyte temperature, the

deposition rate and Ir content increases, whereas the

grain size of Pt–Ir alloy coatings decreases. Dense and

smooth Pt–Ir alloy ¯lms were obtained at 1Adm�2

and 80�C. Pt–Ir alloy coatings with expected compo-

sitions can be readily fabricated by controlling the

mole concentration ratios of [Ir3þ]/[Ir3þ]þ[PtCl2�6 ] in

the electrolyte. Although detailed investigation on the

structure and morphology of electrodeposited Pt–Ir

coatings was also presented in their report, XRD

analysis revealed that all the coated Pt–Ir ¯lms had a

single phase with fcc structure, and the lattice para-

meters of the coatings decrease linearly with increasing

Ir content. The electrochemical reductions through

depositing the alloy are as:

PtCl 2�6 ðaqÞ þ 4e� ! PtðsÞ þ 6Cl�ðaqÞ;E0 ¼ 0:744V

and

Ir3þðaqÞ þ 3e� ! IrðsÞ; E0 ¼ 1:156V:

Table 2. Brief summary of Pt plating electrolyte and conditions.

Types of bath and composition Substrate Plating conditions Thickness Refs.

Chloroplatinic acid, H2PtCl6 � � � 6H2O — 2.5–3.5Adm�2, 15–20% e±ciency — 21

(NH4)2PtCl6 with sodium citrate andammonium chloride

— 2.5–3.5 Adm�2, 45–90�C, up to70% e±ciency

— 20

Dinitrodiammineplatinum, Pt(NO2)2(NH3)2 — Max. 5Adm�2, 70–90�C 7.5�m 22–24

Dinitrodiammineplatinum, Pt(NO2)2(NH3)2 Rene-80 0.3–0.5 Adm�2, 91�C, pH of 10–10.5 and Pt conc. greater than12 gl�

3–5.5�m 26

Dinitrodiammineplatinum, Pt(NO2)2(NH3)2 Inconel-713LC 0.1Adm�2, 15 gl� Pt salt conc. 3–7�m 3 and 18


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Regarding the above reduction potentials, it is ob-

vious that iridium can be preferentially reduced to

platinum, as it is indeed observed in this investigation.

The process is categorized as normal codeposition.45 In

another report, researchers electroplated Pt–Ir alloys

on the nickel-based single crystal superalloy TMS-82

by direct current technique.8 Figure 3 represents dif-

ferent morphologies of Pt–Ir electrodeposits at various

current densities and di®erent plating times. In this

study, preferential deposition of Pt occurred. The

mentioned study utilized amidosulfuric acid solutions

in order to perform alloy plating.

Similar electrolytes were also employed elsewhere

for thin ¯lm Pt–Ir plating,46,47 where it was found

that when Ir content exceeds 3 at.%, Pt–Ir ¯lms

exhibited the (111) preferred growth orientation. It

was also found that pre-deposition of Ni and Pt

promotes the deposition rate and changes composi-

tion of overlaying Pt–Ir ¯lms. The surface of Pt–Ir

deposits was granular consisting of numerous nodules

with mean size increased with deposition time and

current density. Ir addition to Pt-modi¯ed aluminide

coating can be among the solutions to retard surface

rumpling. It was reported that Pt-modi¯ed samples

with Ir content had a much smoother surface than the

Pt-modi¯ed coatings after cyclic oxidation, where the

latter su®ered from severe surface rumpling. In this

regard, when the Ir content exceeded 80 at.% in Pt–

Ir-modi¯ed coatings, internal voids are formed during

cyclic oxidation. Therefore, addition of 30–50 at.% of

Ir to Pt-modi¯ed aluminized coating is most e®ective

in enhancing oxidation resistance.48 Moreover, it was

reported that Pt–Ir-modi¯ed aluminide coating

enhances the type I hot corrosion resistance. Reports

showed that factors including phase transformation

from �-(Ni, Pt)Al to � 0-(Ni, Pt)3Al, characteristics of

the scale and protection by Pt/Ir-enriched layer have

important e®ects on hot corrosion behavior of modi-

¯ed aluminide coatings.49 Here, researchers employ

binary systems since the Pt-modi¯ed aluminide bond

coats are prone to rumpling, during which the ther-

mally grown oxide (TGO) can initiate cracking,

leading to spallation of the thermal barrier.50–54

To improve the oxidation resistance of Pt-modi-

¯ed aluminide coatings, layered structure of PGMs

such as Pt/Pd (Fig. 4)18,55–57 and Pt/Ru (see

Refs. 58–60) were electroplated on superalloys. Pt/

Pd-modi¯ed aluminide coatings also showed superior

resistance to cyclic oxidation and a less rumpled

surface than Pt-modi¯ed aluminide coating after cy-

clic oxidation.55 Moreover, Ru addition to Pt-modi-

¯ed aluminide coating is of another interest, because

Ru additions to Ni-based superalloys have been

shown to improve creep strength.58 Pt/Ru-modi¯ed

bond coating consisting of 2�m Pt þ 2�m Ru was

deposited on a nickel-based superalloy by electrode-

position method, followed by conventional Al pack

cementation by Song et al.59 The PTP-10 solution

used for Pt electrodeposition contained 4–6 gl� Pt,

1 gl� cinnamic acid and 20 gl� NaOH. The tempera-

ture for Pt plating was controlled at 75–85�C, and the

cathodic current density was 0.5–1.5Adm�2. The Ru

(a) (b)

Fig. 2. (a) Dependence of composition and deposition rate of Pt–Ir alloy coatings on current density, Td ¼ 80�C. (b)Dependence of composition and deposition rate of Pt–Ir alloy coatings on electrolyte temperature, id ¼ 1Adm�2, [Ir3þ]/([Ir3þ]þ[PtCl2�6 ])¼ 0.15. Source: Reprinted with permission from Wu et al.6 Copyright: Elsevier (2004).


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plating bath contained 4–6 gl� ruthenium chloride

and 50 gl� sodium sulfamate (NH2OSO2Na).59 The

whole Ru plating process was performed at 50–60�C,where the cathodic current density was 1.5–5Adm�2.

Researchers reported that Ru addition increases

the strength of coatings and suppresses the rumpling

behavior. The absence of rumpling may be res-

ponsible for the improved corrosion resistance of

(a) C.D.: 0.55 A/dm2, 0.5 h (b) C.D.: 0.55 A/dm2, 1 h

(c) C.D.: 0.55 A/dm2, 2 h (d) C.D.: 0.55 A/dm2, 4 h

(e) C.D.: 0.20 A/dm2, 2 h (f) C.D.: 1.00 A/dm2, 2 h

Fig. 3. Surface morphologies of Pt–Ir ¯lms deposited on superalloy TMS-82þ at di®erent current densities and depositiontimes ([Ir3þ]/[Ir3þ]þ[PtCl2�6 � ¼ 0:95). Source: Reprinted with permission from Wu et al.8 Copyright: The Japan Institute ofMetals (2005).


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Pt/Ru-modi¯ed aluminide coating. Wang et al.61

stated that the (Ru, Ni)Al layer (Fig. 5) e®ectively

slows down inward di®usion of Al from the coating

and outward di®usion of alloy elements such as W

and Mo, then suppressing the formation of topologi-

cally close-packed phases and secondary reaction

zone. Table 3 represents the brief summary of depo-

sition electrolyte and conditions for Pt alloy and

layered Pt deposition.

5. Palladium Electrodeposition

Palladium coatings have technological importance, as

palladium exhibits many desirable characteristics

such as excellent tarnishing, wear and corrosion re-

sistance with low electrical contact resistance and

hence a wide range of applications. Palladium is

deposited from many bath compositions based on

simple salt PdCl2 and also from complexes such as

Pd(NH3)2(NO2)2, Pd(NH3)2Br2, H2PdCl4, Pd(NO3)2,

Na2Pd(NO3)4, Pd(NH3)4(NO3)2, Pd(NH3)2(NO3)2and Pd(NH3)4Cl2. Ammonia is the most suitable

complexing ligand for palladium electrodeposition

and most of the available literature is on ammonia

complexed to palladium solely or in combination with

other ligands.20 However, regarding Pd deposition on

superalloy substrates, only a few types of baths have

been employed by researchers. Palladium was plated

from a bath containing 15 gl� Pd(NH3)4Cl2 and

75 gl� NH4Cl (adjusted to pH of 8 with ammonia)

onto two types of superalloys (SRR99 and Rene80).

Using oxidation and hot corrosion tests, researchers

concluded that palladium-modi¯ed aluminide coat-

ings show a high resistance to high-temperature oxi-

dation and to hot corrosion. They can thus be

Fig. 4. SEM images of Pt/Pd layers plated on Inconel-738LC. Source: Reprinted with permission from Honget al.55 Copyright: Elsevier (2009).

Table 3. Brief summary of Pt alloy and Pt layer structures.

Alloy Bath constituents Substrate Plating conditions Thickness Refs.

Pt–Ir IrCl3 � � � 4H3O, H2PtCl6,HOSO2NH2, NaNO3

TMS-75, pH of 2 (using 2N HCl or 1N

NaOH), 80�C, 1Adm�2

2–8�m 6 and 9

Pt–Ir IrCl3 � � � nH2O, H2PtCl6,NH2SO3H, HCl, HNO3

TMS-82 pH of 2–3 (using HCl or NaOH),

80–85�C, 0.2–2Adm�2

3–5�m 8, 46 and 47

Pt/Pd Commercial PlatinK solution/Commercial Alpadin 300solution

738LC At 50�C and 1Adm�2/at 55�Cand 1Adm�2

4–5�m/4–5�m

55–57

Pt/Ru 4–6 gl� Pt, 1 gl� cinnamic acidand 20 gl� NaOH/4–6 gl� Ruand 50 gl� sodium sulfamate

DZ125 75–85�C, 5–1.5Adm�2/50–60�C,1.5–5Adm�2

2�m/2�m 58–60

Fig. 5. BSE image of the cross-section of (Ru, Ni)Al/NiAlcoating on a superalloy after 2 h heat treatment in vacuumat 1050�C. Source: Reprinted with permission from Wanget al..61 Copyright: Elsevier (2011).


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considered as a feasible low-cost high-performance

alternative to platinum aluminides.62 In another re-

search work, Pd was electrodeposited on superalloy

M38 by He et al.63 An electrolyte comprising 20–

30 gl� Pd(NH3)2Cl2, 40-60mlL� NH3 � � �H2O, and

15–25 gl� NH4Cl with respective pH, temperature

and current density of 9.0–9.5, 15–50�C and 0.4–

0.6Adm�2 were used in their work. Based on oxida-

tion test results, they concluded that Pd-modi¯ed

aluminizing coatings are obviously better than con-

ventional aluminide coatings and as good as com-

mercial platinum-modi¯ed coatings under the same

conditions. They also stated that palladium not only

stabilizes the protective alumina formed in cyclic

oxidation and dramatically increases the hot corro-

sion resistance of the coatings, but also increases the

lifetime of the �-phase by impeding the outward dif-

fusion of substrate elements. Palladium with 3 and

7�m thicknesses was plated using the electrodeposi-

tion process on Inconel-713LC superalloy.18,64 (in

the bath of palladium chloride PdCl2: 10 gcm�3, sul-

famates acid H2NSO3: 100 gcm�3, hydrochloric acid

HCl: 20 gcm�3 and ammonium chloride NH4Cl:

50 gcm�3 at 35�C). In another work, to plate palla-

dium on Inconel-738LC, Pd(NH3)Cl2 was used as the

electrodeposition bath at room temperature by Bai

et al. The thickness of the palladium layer was ap-

proximately 2–4�m, with current density of 3Adm�2

for 3min, while coatings have good adherence to the

substrate without any blister and pores.7 Some com-

mercial baths were also employed for electrodeposi-

tion of Pd in the literature.55 Table 4 represents a

brief summary of palladium plating electrolytes and

conditions on di®erent superalloys substrates.

6. Ir, Re and Rh Electrodeposition

IrBr3 is generally reported as the most useful starting

material for iridium deposition, as good deposits can

be obtained from these electrolytes65 at a current den-

sity of 0.1–0.2Adm�2, pH around 1.8 with current e±-

ciency of 30–65%. A pure Ir layer was electrodeposited

on the nickel-based single crystal superalloy TMS-75.9

The basic electrolyte used for the pure Ir electrodeposi-

tion is composed of 4–12g/L IrCl3 � � � 4H2O and 35–

50g/L HOSO2NH2 . The electrodeposition was con-

ducted at temperature of 353K and pH value 2.0 with

0.1�m/min rate. Iridium-modi¯ed aluminide coatings9

show reasonable cyclic thermal oxidation probably be-

cause of Ir presence in the coating, which not only

improves the adherence of oxide scale but also delays the

degradation of �-(Ir, Ni)Al phase to the � 0-phase duringoxidation. Some researchers66 reported that addition of

Ir to aluminide improves the protective performance of

the aluminides by increasing the surface concentration

of Al to approximately 70 at.% and retarding the inter-

di®usion of the alloying elements. Ir–Pt alloy has

been successfully electroplated on an Ni-based single

crystal superalloy TMS-75 by Murakami et al.67

The electrolytes were prepared from amidosulfuric

acid (HOSO2NH2: 38–48 gl�), iridium chloride

(IrCl3 � � � 4H2O: 40 gl�) and chloroplatinic acid hexa-

hydrate (H2PtCl6 � � � 6H2O: 55.5 gl�), where sodium

nitrate (NaNO3: 40–50 gl�) was added to improve

their conductivity. The bath pH was adjusted using

2N HCl or 1N NaOH and the cathodic current density

was set as 0.5–4.0Adm�2. In connection with oxida-

tion test results of Ir–Pt-modi¯ed aluminide coatings,

authors reported that these coatings are promising for

replacing the conventional Pt–Al ones because Ir may

Table 4. Brief summary of palladium plating electrolyte and conditions.

Types of bath and composition Substrate Plating conditions Thickness Ref.

15 gl� Pd(NH3)4Cl2, 75 gl�

NH4ClSRR99 pH of 8 — 62

20–30 gl� Pd(NH3)2Cl2,40–60mlL� NH3 � � �H2O, and15–25 gl� NH4Cl,

M38 pH of 9–9.5, 15–50�C and

4–6mAdm�2

8�m 63

PdCl2 10 gcm�3, H2NSO3

100 gcm�3, HCl 20 gcm�3 and

NH4Cl 50 gcm�3

713LC At 35�C 3–7�m 18 and 64

Solution comprising Pd(NH3)Cl2 Inconel-738LC Current density of 3Adm�2 for 3min 2–4�m 7


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play a role as solid solution strengthener and does not

deteriorate the creep properties of substrates.67

Re electrodeposition was also carried out by some

authors on Ni-based superalloys K3,61 TMS-8268 and

Hastelloy X.68 In another study,69 MCNG Ni-based

superalloy substrate was coated by electrolytic Re

using a solution comprising 23 gl� ReO�4 , 30 g/L

NH4F, 10 gl� (NH4)2SO4 and H2SO4 to adjust pH at

1. It is reported that the Re deposit obtained by this

process are 2�m thick, dense and well adhered, with

some cracks due to internal stresses developed during

deposition. Good interdi®usion characteristics are

also attributed to these Re layers during thermal

treatments.68,69

Bai et al.7 electroplated Rh on Inconel-738LC

using an electrolyte comprising Rh2(SO4)3 at room

temperature with a current density of 3Adm�2 for

5min. The thickness of the Rh layer was reported

approximately as 2–3�m, where the coatings have

good adherence to the substrate without any blisters

and pores. They used Re deposits for modi¯cation of

aluminide coating. Based on XRD and EPMA anal-

yses, after oxidation tests of Rh-modi¯ed aluminide,

Bai et al. found that a continuous thin oxide scale of

compact Al2O3 was formed at the surface of Rh–Al

specimen and adhered to the substrate after the cyclic

oxidation test. This oxide layer indicated no cracks

and it e®ectively prevented the substrate from oxi-

dation under the unfavorable condition of cyclic

thermal stresses. Moreover, it was reported the Rh

remains within the oxide scale and substrate serves as

a barrier against the outward depletion of aluminum

atoms in the cyclic oxidation process.7

7. Ni and Ni Alloy Electrodeposition

Although electrodeposition of nickel is very common,

there are few reports related to electrodeposition of

nickel on superalloys substrates. Nickel with 2�m

thickness was electrochemically plated from sulfate–

chloride electrolyte on superalloy M38. An electrolyte

consisting of 250–300 gl� NiSO4 � � � 7H2O, 30–60 gl�

NiCl2 � � � 6H2O, 35–40 gl� H3BO3 (the famous Watts

solution) with respective pH, temperature, and

cathodic current density of 3–4, 45–60�C, and 1–

2.5Adm�2 were used as well as a nickel sheet for anode

material. Narita et al. electroplated pure 5�m thick

Ni from Watts solution at a temperature of 70�C on

TMS-82t and Hastelloy X. Before electrodeposition,

they ground the specimens with 150-grit water-proof

sand paper, followed by degreasing them in a metha-

nol–benzene solution under ultrasonic agitation.68 In

another research, Watts nickel bath (NiSO4� 6H2O:

0.95molL�; NiCl2� 6H2O: 0.17molL�; H3BO3:

0.65molL�) was used for Ni electrodeposition on su-

peralloy TMS-82t.8 Some researchers70 plated Ni on

Inconel-718SPF using electroless process. Since elec-

troless nickel plating is an autocatalytic reaction pro-

cess, the nickel ions (Ni2þ) existing in a nickel

electroless bath react with hypophosphorus ions

(H2PO�2 ) to form nickel atoms. Meanwhile, the hypo-

phosphorus ions react with nascent hydrogen to form

phosphorus atoms. The nominal reaction is

Niþ2i þ H2PO

�2 ! Nii þ 2Hþ þ H2PO

�3 :

Electrodeposition of Pd–Ni was successfully carried

out using an electrolyte comprising 15–45 gl�

Pd(NH3)2Cl2, 30–50 gl� NiSO4 � � � 7H2O, 70–

100mlL� NH3 � � �H2O and 40–60 gl� NH4Cl.63 In ad-

dition, Pd–20%Ni coating was applied on superalloy

M38 with plating conditions at pH of 8.5–9, tempera-

ture of 15–50�C and 0.8–1.2Adm�2. Narita and cow-

orkers also used Pt as an anode material, where they

electroplated a 10�m-thick Re–Ni alloy ¯lm contain-

ing approximately 70 at.% Re from a pH of 3 aqueous

solution using an Ni anode at a temperature of 50�C.The solution contained ReO�

4 (0.1molL�), nickel sul-fate (0.1molL�) and citric acid (0.1molL�). They

used TMS-82t and Haselloy X in their research as

substrates.

Cavaletti et al.71 electrodeposited Ni–W coatings

as a di®usion barrier to limit interdi®usion between

an Ni-based superalloy MCNG and a �-NiAl bond

coat. Ni–W coatings were plated from an electrolytic

bath containing 20 gl� NiSO4 � � � 7H2O, 100 gl�

NaWO4 � � � 2H2O and 66 gl� C6H8O7 � � �H2O. The

respective pH, temperature and current density of the

bath were ¯xed at 7.5 and 70�C and 15Adm�2.

Authors reported preparation of 7�m–thick Ni–W

coating with a composition of Ni (75–80 at.%) and W

(20–25 at.%) through 10min deposition. Along with

electrodeposition of Ni–W on superalloy substrate,

they reported that W-rich layer formed with Ni–W

coating modi¯es the oxidation behavior of the bond

coat and limits inter-di®usion of substrate alloying

elements. In another work Mercier et al.69 electro-

plated 7�m-thick Ni–W electrolytic layer on as


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Rhenium-deposited Ni-based superalloy (MCNG)

from above mentioned electrolytic bath. Regarding

the obtained coating some authors reported that the

combination of Re and Ni–W electrolytic deposits

demonstrates satisfactory thermal characteristics.

8. Composite Electrodeposition

Ouyang et al.72 electrodeposited Ni–BaCr2O4 com-

posite coatings (Fig. 6) on superalloy Inconel-718 from

a Watts-type electrolyte containing BaCr2O4 parti-

cles, dispersed in suspension with the aid of a cationic

surfactant cetyltrimethylammonium bromide

(CTAB). They reported the optimum electrodeposi-

tion parameters as 70 gl� of BaCr2O4 powder concen-

tration in solution, temperature of 50�C and 2Adm�2

of current density, where CTABused as additive in the

electrolyte was with a concentration of 0.05 gl�. TheBaCr2O4 content in composite coating was reported as

16.6 vol.% under optimum electrodepositing condition.

This composite coating exhibited a relatively smooth

surface and uniform distribution of BaCr2O4 particles.

The authors demonstrated that the content of

BaCr2O4 incorporated into composite coatings

depends mainly upon the BaCr2O4 powder concen-

tration in bath, cathode current density and temper-

ature of electrolyte. They also investigated the friction

and wear properties of Ni–BaCr2O4 composite coat-

ings with a ball-on-disk friction and wear tester in

sliding against alumina ball. Their results showed that

the wear resistance of Ni–BaCr2O4 composite coating

electrodeposited on superalloy Inconel-718 increases

with increasing the BaCr2O4 content.

Since electrodeposition of MCrAlY coatings is a

new attempt involving easy and cost-e®ective appli-

cation of overlay or bond coatings on the hot section

parts of gas turbines, NiCrAlY coating was applied to

nickel-based superalloy Inconel-738 using electrolytic

deposition by Saremi and Bahraini.19 Before plating,

the samples were treated anodically in concentrated

(65–70%) H2SO4 at room temperature and at

20Acm�2 for 1min based on ASTM B558. To obtain

NiCrAlY coating, nickel was electroplated using a

Watts bath containing aluminum (5 gl�) and chro-

mium (25 gl�) particles, where yttrium was applied

by hot-doping. The electrodeposition parameters for

NiCrAl deposition were the current density of 5–

7Adm�2, temperature of 50–65�C, pH of 4 and

plating time of 20–30min. Here, heat treatment was

also used to homogenize the coating. It was reported

that all four elements are present in the coatings

within the desired concentration range, as well as two

phases of �-Cr and �-NiAl. The hot corrosion resis-

tance of the coating was studied by an electrochemical

test method, where a notable performance of such a

coating at high anodic polarization was observed. As

the adhesion of the coating during 100 h of cyclic ox-

idation was acceptable, the hot corrosion and oxida-

tion resistance consequently show a signi¯cant

improvement. Electrodeposition of Ni(Zr) composite

coating on Cr-pack-cemented Ni-based superalloy

TMS-82t was also reported elsewhere.68 Ni(Zr) ¯lm

was coated using an Ni Watts solution containing

6 gl� Zr powder solution, under a current density of

1.5Adm�2, where the average size of the Zr powder

was 3�m. The Ni(Zr) composite electroplated speci-

men was heat treated in the temperature range of

(a) (b)

Fig. 6. Surface morphologies of pure nickel coating and Ni–16.60 vol.% BaCr2O4 composite coating: (a) Pure nickel coating;and (b) Ni–16.6 vol.% BaCr2O4 composite coating. Source: Reprinted with permission from Ouyang et al.72 Copyright:Elsevier (2013).


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800–1150�C in a vacuum of 10–3Pa to allow Zr be-

coming evenly distributed in the composite coating.

AnNi-based composite plating bathwas patented by

Grossman and Grylls.73 Their invention provided a

method to coat the superalloy substrate using a plating

process, either electroless or electrolytic, wherebyNi, Co,

Fe or combinations thereof (but preferably Ni) was

plated on the superalloy substrate using a solution that

included a suspension of powders or containing one or

more of the following elements: Ni, Cr, Al, Zr, Hf, Ti, Ta,

Si, Ca, Fe, Y and Ga. It was also reported in their in-

vention that the optional heat treatment at a tempera-

ture above 870�C and time between 30min and 150min

are su±cient to allow di®usion between the elements

comprising the entrapped powders and the matrix.

9. Concluding Remarks

Electrodeposition of metals, alloys and composites on

superalloy substrates was successfully carried out to

improve and modify the oxidation and hot corrosion

behavior of high-temperature coatings applied widely

on gas-turbine engine components. These electro-

deposited ¯lms are usually deposited in the range of

2–7�m thickness. The important factor in almost all

of these electroplated coatings is the di®usion treat-

ment. Applying di®usion treatment to the coatings

provides their good adherence to the superalloys.

Since superalloys are passivated by the formation of

surface oxide ¯lms, which reduces adhesion of plated

¯lms, special cleaning and processing are required to

remove these ¯lms prior to electrodeposition.

To stabilize �-phase and delay transformation of

�-phase to � 0-phase, platinum and its alloys were elec-

troplated before aluminizing. P- and Q-electrolyte are

used extensively for electrodeposition of Pt on super-

alloys. Platinumand layered structures of Pd, Ir, Ru and

Re were electrodeposited on di®erent superalloys. Nickel

and nickel alloys were also plated using either electro-

deposition or electroless methods. The well-known

Watts bath was utilized for nickel plating. Furthermore,

NiCrAlY, Ni–BaCr2O4 and Ni(Zr) coatings were ap-

plied as composite coatings on Ni-based superalloys.

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Documents

ELECTRODEPOSITION ON SUPERALLOY SUBSTRATES: A REVIEW · source of Pt with sodium citrate and ammonium chloride as supporting electrolyte and complexing agent, the current e±ciency