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An Electroplating of Structural P/M Parts – Medium Density Steel Parts
M.C.F.Ierardi1, C.A. Siviero Filho1, D.T.A. Figueira Filho2
1
Departamento de Materiais e Processos de Fabricação, Faculdade de Engenharia Mecânica,
Universidade Estadual de Campinas, CEP 13083-970, Campinas-SP, Brasil
clara1@fem.unicamp.br / siviero@fem.unicamp.br
2
Departamento de Metalurgia e Materiais, Centro Universitário da FEI, CEP 09850-901,
São Bernardo do Campo-SP, Brasil
sinter@sti.com.br
Keywords: Powder Metallurgy, Electroplating, Secondary operation, Porosity, Impregnation.
Abstract: Surface treatment processes, including electroless plating and electroplating, can be
applied to structural ferrous P/M parts to obtain better surface properties, such as wear
resistance, corrosion resistance and appearance. These finishing processes carried out as a
secondary operation are essential for the quality and performance of the product.
Electroplating is usually realized on P/M parts as well as on conventional materials.
Differences arise because the metal powder part is porous. Relevant aspects of the porous
structure of medium-density steel parts (5,4 to 6,6 g/cm3) have been investigated. Plating
solutions can penetrate in interconnected pores and cause the last effect, known as
"flowering". These problems can be overcome by impregnation of porosity. Details of
porosity and its sealing were investigated.
Introduction
Powder Metallurgy (P/M), also referred to as Sintering, is a highly developed method
of manufacturing reliable ferrous and nonferrous parts. Made by mixing elemental or alloy
powders and compacting the mixture in a die, the resultant shapes are then sintered or heated
in a controlled-atmosphere furnace to bond the particles by metallurgical process[1]. P/M is
essentially a forming process. The part does not always emerge ready for use at the end of the
sintering step even though it has acquired a certain level of mechanical properties. Just as with
alternative technologies it is possible to modify some of these properties, to improve the
shape or the dimensional accuracy so as to extend the range of application [2].
One of the available technologies that can be used to improve resistance to corrosion
of elements produced by P/M is the surface coatings [3]. In this context, electroplating of P/M
compacts carried out as a secondary operation following compacting and sintering is essential
for the quality of the product. This technique is usually performed on compacts in the same
manner it is performed on conventional materials, although the plating process often needs to
be modified because of the porous structure of the substrate. Considering the corrosion
process the volume of open porosity is the essential parameter. Porosity of low-density
samples consists almost entirely of open pores, whereas samples with high-density contain
more insulated than open pores [4]. In addition, porosity limits the quality of coatings that can
be obtained. As a result, it is not advisable to apply any type of coating on samples produced
by powder metallurgy [5]. In this manner, the uniformity and homogeneity of coatings is very
important by sealing of porosity. So, the sealing of porosity is very important for uniformity
and homogeneity of coatings.
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The present work investigates relevant aspects of the porous structure of mediumdensity steel parts (5,4 to 6,6 g/cm3) and its sealing by electroplating of copper and tin as an
forward step to zinc and zinc-nickel deposits.
Experimental Procedure
Iron-cooper-graphite P/m plain cylindrical bearings with density between 5,4 to
6,6g/cm³ were used as substrate for electroplating. The iron compacted is based on atomized
iron powder (Höganäs AHC100.29) with 4% copper powder (150µm), 0,8% graphite and
0,8% zinc stearate, compacted and sintered at 1090°C on 90N210H2 atmosphere.
After that, the parts were subdued to metallografic and characterized by the image
acquisition program Q500MC Leica coupled to the optical microscope Zeiss Neophot 32.
First a 10-time-amplified macroanalysis of each sample was done through the division of the
transversal section in three regions, in order to study the compacting process and porosity
distribution. Later a 200-time-amplified microanalysis was carried out in order to verify the
interaction between the galvanic coating and the sintered sample.
The electrolytical coating deposit of zinc and zinc-nickel was obtained through
processes and techniques used for conventional materials. Methods were studied in order to
come to a coating of good cover and appearance, free of migration products (electrolyte),
characteristic that would grant resistance to long-term corrosion. So, during the phase of pretreatment, the superficial oxidation of the parts was removed by blasting with glass spheres;
then the parts were washed in cold water and degreased anodically. In an attempt to seal the
pores without using resin or wax, copper coating was applied by alkaline process and tin was
deposited by acid process.
The choice of the electrolytical sealing is based in the concept that the whole process
is restricted to the environment of conventional galvanic superficial treatment. In addition, tin
was only applied for the production of the zinc coatings by acid process, as an afterward step
to copper electroplating and forward step to alkaline zinc plating. For the zinc alloy, the level
of nickel was determined by the scanning microscope Jeol, model JXA-840B.
Results and Discussion
Figure 1 below shows the optical micrography of the sample with less density
(5,4g/cm³) and higher level of total porosity (31%). Figure 2 shows the optical micrography of
the sample with higher density (6,6g/cm³) and lower level of porosity (16%). Samples with
intermediate density were also part of this study, with variation of 0,2 g/cm³ (5,6; 5,8; 6,0;
6,2; 6,4g/cm³).
It is important to stress that in the density limits applied here, the level of open
porosity (open pores, interconnected and connect to surface of the part) means 80% or more
of total porosity, favouring electrolyte retention, which affect the subsequent plating.
In all the samples studied, porosity distribution between the center and the most distant
areas were in acceptable conditions.
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(c)
Figure 1. Optical micrography belonging to the sample with density 5,4g/cm³ (amplified 10x,
without chemical attack). a) higher extreme area; b) central area; c) lower extreme area.
(a)
(b)
(c)
Figure 2. Optical micrography belonging to the sample with density 6,6g/cm³ (amplified 10x,
without chemical attack). a) higher extreme area; b) central area; c) lower extreme area.
Electroplating
Zinc plating
Table 1 summarizes the steps taken during each of the routes with zinc plating for
samples obtained from different levels of porosity.
Table 1. Zinc plating development.
Processes
Route 1
Blasting
Degreasing
Alkaline cooper
Acid tin
Alkaline zinc
Acid zinc
White passivation
Route 2
Route 3
Route 4
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
For routes 1 and 2 the samples oxidized immediately after passivation, because the
alkaline zinc coating layer was not thick enough for passivation. In terms of quality, the
deposits obtained through routes 3 and 4 presented good cover and appearance after
passivation. However, after one week, white and red corrosion appeared, whose intensity
increased as density decreased. In route 4, when tin coating was added to the process, there
was a considerable reduction in the appearing of corrosion products, especially in densities
6,4 and 6,6g/cm³, when only white corrosion was limited to the area of neutral zone.
Electroplating time: anodic degreasing (strike), alkaline cooper (1min), acid tin (10min),
alkaline zinc (2min) and acid zinc (15 min).
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Figure 3 shows the zinc coating for density 6,6g/cm³, according to development
described by route 4. Total thickness of layer is 20,13±2,51µm. Copper and tin deposits
follow the shape of the part in an attempt to seal the pores. The zinc coating, however, was
very porous, which favors the communication between the outer environment and the open
porosity of the substrate, as well as migration of red corrosion.
Figure 3. Zinc deposit, route 4.
Table 2 summarizes the steps taken during each of the routes with zinc-nickel plating
for samples obtained from different levels of porosity.
Table 2. Zinc-nickel plating development.
Processes
Route 5
Route 6
X
X
X
X
X
X
X
Blasting
Degreasing
Alkaline cooper
Zinc-nickel
Passivation
X
X
These 2 routs are only different in what the alkaline copper was added to the process
in route 6. In terms of quality, the addition of copper was allowed for the development of
layers with better cover and appearance. Electroplating time: anodic degreasing (strike),
alkaline cooper (3min) and zinc-nickel (15 min).
Figure 4 shows the development of the zinc-nickel plating for density 6,6g/cm³,
according to route 6. Thickness is 6,21±1,0µm for the copper layer and 9,81±1,61 for Zn-Ni
alloy. The alloy coating (12% of nickel) was considered a good covering, without pores, and
only microfissures could be observed. The increase of the copper layer allowed a more
effective sealing as shown in figure 6.
Figure 4. Zn-Ni deposit, route 6.
Figure 5. Cooper deposit following the
exposed open pore surface, near the surface.
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Conclusions
•
•
•
•
The pre-treatment of anodic degreasing, with strike copper deposit in the form of thin
film, helps considerably the afterward by copper or tin deposit, replacing occasional
resin impregnation;
Electroplating of copper showed sealing properties towards the acceptable porosity,
adjusting to the surface of the sample;
Electroplating of tin showed good sealing properties only for P/M compacts with
density up to 6,4g/cm³;
Zinc-nickel deposit showed greater ability of covering, when compared to zinc
coating, which favors resistance to corrosion.
References
[1] Metal Powder Industries Federation (MPFI). P/M Design Guidebook, New Jersey, USA,
(1983).
[2] E. Mosca. Powder Metallurgy: Criteria for Design and Inspection. AMMA-Associazione
Industriali Metallurgici Meccanici Affini, Turim, Italia, (1984).
[3] W.Riedel. Electroless nickel plating. ASM international and Finishing Publications, Ohio,
USA, (1998)
[4] P.Leisner, R.C.Leu and P.Moller. Eletroplating of porous PM compacts. Powder
Metallurgy, v.40, no.5, (1997), pp.207, 207-210.
[5] A.M.Bolarín, F.Sánches, A.Barba, O.Coreño and J.Coreño. Electroless nickel plating of
atomised and sponge iron compacts. Surface Engineering, v.19, no.5, (2003), pp.364, 364368.
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