Intermediary phases Ni-Al and Fe-Al are promising materials due to their superior properties such as hardness and good resistance against oxidation at high temperatures. Moreover, Fe-Al phases are resistant in sulphur - containing atmospheres. Because of these characteristics, the above mentioned intermetallic phases seem to be prospective for the use in many technical applications such as energetics, chemical or automotive industry in a form of a bulk material or coatings. Presently, the protective aluminide layer is usually prepared by thermal spraying. Nevertheless, this method is not suitable for complex-shaped components. Therefore, the aim of this work was to find an alternative way to prepare layers consisting of nickel or iron aluminides by other technique than thermal spraying. At first, carbon steel samples were coated using galvanic or electroless nickel plating. Coated samples were subsequently submerged into molten aluminium at various temperatures and process durations.  The influence of the temperature and duration on the intermetallic phase growth was studied by scanning electron and light microscopy. Thickness and microhardness of the intermetallic layer was also measured

Novák, Pavel

Voderova, Milena

English

Intermediary phases Ni-Al and Fe-Al are promising materials due to their superior properties such as hardness and good resistance against oxidation at high temperatures. Moreover, Fe-Al phases are resistant in sulphur - containing atmospheres. Because of these characteristics, the above mentioned intermetallic phases seem to be prospective for the use in many technical applications such as energetics, chemical or automotive industry in a form of a bulk material or coatings. Presently, the protective aluminide layer is usually prepared by thermal spraying. Nevertheless, this method is not suitable for complex-shaped components. Therefore, the aim of this work was to find an alternative way to prepare layers consisting of nickel or iron aluminides by other technique than thermal spraying. At first, carbon steel samples were coated using galvanic or electroless nickel plating. Coated samples were subsequently submerged into molten aluminium at various temperatures and process durations.  The influence of the temperature and duration on the intermetallic phase growth was studied by scanning electron and light microscopy. Thickness and microhardness of the intermetallic layer was also measured.</p

Milena Voderova

Pavel Novák

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PROTECTIVE LAYERS OF IRON AND NICKEL ALUMINIDES ON STEEL

Materials Engineering - Materiálové inžinierstvo (MEMI)

M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
112 
PROTECTIVE LAYERS OF IRON AND NICKEL 
ALUMINIDES ON STEEL 
 
Milena Voděrová 1,*, Pavel Novák 1 
 
1
 Department of Metals and Corrosion Engineering, ICT Prague, Technická 5, 166 28 Prague, Czech Republic  
* 
corresponding author: tel. +420 220 445 026, e-mail: voderovm@vscht.cz 
 
 
Resume 
Intermediary phases Ni-Al and Fe-Al are promising materials due to their 
superior properties such as hardness and good resistance against oxidation at 
high temperatures. Moreover, Fe-Al phases are resistant in sulphur - containing 
atmospheres. Because of these characteristics, the above mentioned intermetallic 
phases seem to be prospective for the use in many technical applications such as 
energetics, chemical or automotive industry in a form of a bulk material or 
coatings. Presently, the protective aluminide layer is usually prepared by thermal 
spraying. Nevertheless, this method is not suitable for complex-shaped 
components. Therefore, the aim of this work was to find an alternative way to 
prepare layers consisting of nickel or iron aluminides by other technique than 
thermal spraying. At first, carbon steel samples were coated using galvanic or 
electroless nickel plating. Coated samples were subsequently submerged into 
molten aluminium at various temperatures and process durations.  
The influence of the temperature and duration on the intermetallic phase growth 
was studied by scanning electron and light microscopy. Thickness and 
microhardness of the intermetallic layer was also measured. 
 
Available online: http://fstroj.uniza.sk/journal-mi/PDF/2013/15-2013.pdf 
Article info 
 
Article history: 
Received 17 December 2012 
Accepted 30 April 2013 
Online 28 July 2013 
 
 
 
Keywords: 
Aluminide; 
Protective layer; 
Nickel coating; 
Fe2Al5. 
 
 
ISSN 1335-0803 (print version) 
ISSN 1338-6174 (online version) 
 
1. Introduction 
In recent years, intermetallics based on 
aluminium and transition metals, e.g. nickel or 
iron, have been widely studied. Generally, 
aluminides seem to be interesting materials in   
a bulk form or coating due to their superior 
properties, such as low densities, high melting 
points and corrosion resistance. The strength of 
some intermetallics remains satisfactory with 
increasing temperature or can even increase as 
the temperature increases. Mentioned properties 
determine aluminides to be used especially for 
high- temperature applications. Intermetallic 
phases containing Ni and Al can differ from  
Fe-Al in some points. The major advantages of 
nickel aluminides are e.g. resistance to 
oxidation and carburization in both oxidizing 
and reducing atmospheres up to 1100 °C, 
fatigue and wear resistance or superior creep 
strength. [1, 2] Iron aluminides are promising 
while used in sulphur containing atmospheres, 
have high electric resistivity increasing with 
temperature and remain stable in many aqueous 
solutions. Although it seems that aluminides 
have only advantages, their ductility is limited 
because of high hardness and brittleness. 
Manufacturing and processing of intermetallics 
is also limited. As it was mentioned above, due 
to the suitable properties of aluminides 
protective layers formed by Al and Ni or Fe are 
perspective e.g. for components which are 
exposed to elevated temperatures or exhaust 
gases. There are many methods how to prepare 
aluminide layer, for example by thermal 
spraying [3, 4], CVD [5-7] or by submersion of 
the component into molten aluminium. Pre- 
plating of the substrate material, mostly steel, is 
usually realized by galvanic Ni plating [8, 9]. 
Within galvanic nickel plating which is 
commonly used, electroless nickel coating was 
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. 
To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative 
Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA. 
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M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
113 
used in this work to prepare homogeneous 
nickel coating. The main advantage of this 
process is that even complex-shaped materials 
can be coated with nickel of similar thickness 
on all the parts of the component. The only 
problem is ecological disposal of waste 
solution. 
Aim of this work was to find 
an alternative way how to prepare mentioned 
layers economically and simply with accent on 
using it on complex machine parts and 
elements. Replacing thermal spraying process 
by submersion the sample into molten metal 
seems to be perspective. 
2. Experimental 
As a substrate material, plain carbon steel 
was used in this experiment. Steel contained 
0.13 % C, 0.27 % Mn, 0.02 % S. Composition 
of steel was determined by glow discharge 
atomic emission spectrometry (GDS). 
Surface of rectangle samples 
(2.5 x 2 x 0.2 cm) was treated by grinding and 
rinsed by water and ethanol to remove iron 
oxides and impurities. These samples were 
consequently coated with nickel by two 
different procedures - galvanic nickel plating 
and electroless nickel plating. Required 
thickness of nickel layer was approx. 10 μm. 
 
2.1. Galvanic nickel plating 
Galvanic nickel plating was carried out in 
nickel plating solution which consisted of 60 g/l 
NiSO4, 120 g/l Na2SO4 and H2SO4. The pH 
value of the bath was maintained at pH = 3. 
Nickel plating was carried out at 60 °C with 
current density 50 A/m2 and voltage between 
two lead anodes and steel cathode 4.5 V. Time 
of electrolysis was calculated to 10 minutes. 
 
2.2. Electroless nickel plating 
This method is based on charge exchange 
reaction in water solution. Nickel is cemented 
on iron together with a small part of phosphorus 
(= reduction) and hypophosphite changes into 
phosphate (= oxidation). Before plating in this 
nickel bath, the samples were pickled for 
3 minutes in HCl 3 : 1 (acid : water) and then 
three minutes in H2SO4 1 : 3, cleaned by ethanol 
and dried by hot air. Electroless nickel plating 
proceeded in nickel bath which consisted of 
34 g/l Ni(C3H5O3)2, 31,5 g/l Ni(H2PO2)2, 10 ml/l 
C3H6O3 and 5ml of NaOH per every 25 ml of 
solution. Dissolving the content was taken at 
75 °C and coating process was carried out at 
90 °C for 120 minutes. After 40 and 80 minutes 
from start of the procedure, NaOH in required 
amount was put into the bath. Coating prepared by 
electroless plating procedure contained 17 at.% 
phosphorus which was determined by GDS. 
 
2.3. Reaction synthesis of aluminides 
Coated samples were submerged into 
molten aluminium under normal pressure at 700 
and 900 °C for various time periods (30 and 
240 s). As the reference materials, two steel 
samples without nickel coating were introduced 
to the aluminium melt under the same 
conditions. 
As a different variant of the aluminide 
synthesis, one sample coated by galvanic 
process was put into aluminium powder and 
annealed at 650 °C for 8 hours. 
 
2.4. Samples preparation and measurement 
After the coating process had finished, 
samples were air-cooled and metallographic 
samples were prepared as perpendicular cuts 
across the layer. Samples were ground using 
P60- P4000 abrasive papers and polished by D2 
diamond paste. These samples were observed 
by light and electron microscope.  
Thickness of produced aluminide layer 
was measured by image analysis of the light 
micrograph using LUCIA 4.8 image analysis 
software. Microhardness (HV 0.05) of 
aluminide layer was measured by Hannemann 
microhardness tester. 
Chemical composition of aluminides in 
the layer was determined by Tescan Vega 3 
scanning electron microscope using Oxford 
Instruments INCA 350 EDS analyser and 
compared with results obtained in [8]. 
M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
114 
3. Results and discussion 
3.1. Microstructure of aluminide layers 
In Fig. 1, diffusion process during 
submersion in molten aluminium is described. 
Diffusion of aluminium and iron is described by 
arrows. Text labels mark presence of phases 
formed in the early state of submersion. Molten 
aluminium enters the surface of nickel forming 
nickel aluminide Ni3Al and then iron. From the 
surface inwards, iron diffuses into nickel and 
aluminium forming solid solution Fe-Ni and 
different types of iron aluminides- FeAl2, FeAl3, 
Fe2Al5. Iron aluminide formation is probably     
a consequence of the delamination of the 
produced nickel aluminide layer. 
At first, samples produced by                   
a combination of galvanic nickel plating and hot 
dip aluminizing (Ni GAL) were investigated. 
Electron micrographs show the representation 
of various intermetallic phases which originate 
from reactions during submersion. In the case of 
the sample submerged for 30 seconds (Fig. 2) 
there still remain small parts of non-reacted 
nickel. Second nickel-containing phase is NiAl3 
aluminide. Aluminium diffuses towards the 
steel substrate and forms Fe2Al5 as it was 
described in [10]. This phase can be found in all 
the samples submerged for 4 minutes at 700 and 
900 °C (Fig. 3 and 4). The difference is in the 
amount of nickel. There is no nickel in the 
elemental form or in nickel aluminide. Only 
small content of nickel (approx. 1 wt. %) was 
indentified in iron aluminide. Sample 
submerged at 900 °C contained areas of 
aluminide FeAl3, as seen in the picture. 
As well as in the case of samples with 
galvanic coating submerged for 4 minutes, 
layers produced by a combination of the 
electroless nickel plating and aluminizing in the 
melt (Ni-P) exhibit only very low content of 
nickel in the intermetallic layer (Fig. 5, 6). 
Transition zone between iron and aluminium 
consists of aluminide Fe2Al5, sample treated at 
900 °C contained small areas of FeAl2 included 
in Fe2Al5.  
Low nickel content in prepared layers can 
be explained as following: Reaction which 
forms aluminides of nickel is strongly 
exothermic. Due to a different thermal 
expansion coefficient of nickel aluminide and 
steel, the Ni-Al layer delaminates and sinks to 
the bottom of the melting crucible due to higher 
density than aluminium. 
 
 
 
Fig. 1. Diffusion process during submersion. 
 
 
 
Fig. 2. Ni GAL 700 °C / 0.5 min. 
M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
115 
 
Fig. 3. Ni GAL 700 °C / 4 min. 
 
 
Fig. 5. Ni-P 700 °C / 4 min.  
 
 
Fig. 4. Ni GAL 900 °C / 4 min. 
 
 
Fig. 6. Ni-P 900 °C / 4 min. 
 
Due to extreme heat generation of Ni + Al 
reaction, nickel aluminide can be even molten 
and dissolved in the melt. As longer is the 
submerging time that less nickel remains on 
steel. Heat produced by the Ni+Al reaction then 
supports the formation of iron aluminides. 
Samples without nickel coating were submerged 
at 700 °C. Layer originated on sample 
submerged for 0.5 minute consists of a thin 
Fe2Al5 phase. Sample treated for longer time 
period included layer of Fe2Al5, about 50 μm of 
thickness. Third technology – combination of 
galvanic plating and powder aluminizing – 
produced layers of Fe2Al5 of different thickness 
which may depend on amount of powder 
present on surface of steel. 
 
3.2. Thickness and microhardness of aluminide 
layers 
Light micrographs reveal that the 
thickness of the Fe2Al5 layer increases with 
process duration and temperature.  
M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
116 
 
Fig. 7. Fe 700 °C / 0.5 min. 
 
 
Fig.8. Fe 700 °C / 4 min. 
 
Fig. 9. Fe2Al5 made with Al powder. 
 
An increase of the process temperature 
improves the diffusion of aluminium into 
substrate material is much faster and as            
a result, the penetration depth is higher. 
Thickness of the layer used in graphs was 
created as the mean value of 30 values 
measured at different places on the sample 
(Fig. 10). 
Presented microhardness HV 0.05 
(Fig. 11) was averaged from 10 measurements. 
It could be seen that layers prepared at 900 °C 
reach higher values of hardness. It could be 
explained so, that formed aluminide of iron 
Fe2Al5 can exist in wider range of Al and Fe 
concentration. Ratio of iron and aluminium 
forming Fe2Al5 can differ a little bit from 
aluminide originating at 700 °C. If there is little 
more iron included, microhardness can increase. 
Influence of phosphorus in electroless nickel 
coating is discussed. 
 
M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
117 
 
Fig. 10. Thickness of Fe2Al5. 
 
 
Fig. 11. Microhardness of Fe2Al5. 
 
4. Conclusions 
Introduced methods are not suitable for 
producing compact layers containing aluminides 
of nickel. Only in the case of sample submerged 
for 0.5 minute there was a small amount of 
NiAl3 and nickel in elemental form. Because of 
intensive exothermic reaction, originating 
aluminides of nickel delaminate from steel and 
dissolve in molten aluminium. On the other 
hand, these methods are promising for creating 
Fe-Al layers. It was demonstrated that there is 
an influence of temperature and submersion 
duration on thickness and microhardness of 
layer. Samples submerged for the same time at 
higher temperature are characterized by thicker 
layer composed mainly of Fe2Al5 with small 
parts of FeAl2 or FeAl3. Thickness of layer 
varies in range of 30- 200 μm. There is also       
a difference between microhardness of layers 
produced on galvanic and electroless nickel 
coatings. Layers produced at higher 
temperatures are harder because of different 
aluminium- iron ratio in Fe2Al5. There may be 
an influence of phosphorus present in the case 
of electroless nickel coatings, which may form 
hard phosphides. Presence of nickel coating 
may influence kinetics the iron aluminide layer 
formation. Powder aluminizing produced the 
layer formed by Fe2Al5 as in the case of melt 
submersion process, but the laik, yers are less 
homogeneous. 
 
Acknowledgment 
This research was financially supported 
by Czech Science Foundation, project No. 
P108/12/G043 and by Grant Agency of ASCR, 
project No. KAN300100801. 
M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel 
  
 
Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 
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Intermediary phases Ni-Al and Fe-Al are promising materials due to their superior properties such as hardness and good resistance against oxidation at high temperatures. Moreover, Fe-Al phases are resistant in sulphur - containing atmospheres. Because of these characteristics, the above mentioned intermetallic phases seem to be prospective for the use in many technical applications such as energetics, chemical or automotive industry in a form of a bulk material or coatings. Presently, the protective aluminide layer is usually prepared by thermal spraying. Nevertheless, this method is not suitable for complex-shaped components. Therefore, the aim of this work was to find an alternative way to prepare layers consisting of nickel or iron aluminides by other technique than thermal spraying. At first, carbon steel samples were coated using galvanic or electroless nickel plating. Coated samples were subsequently submerged into molten aluminium at various temperatures and process durations. The influence of the temperature and duration on the intermetallic phase growth was studied by scanning electron and light microscopy. Thickness and microhardness of the intermetallic layer was also measured

Milena Voděrová

M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 112 PROTECTIVE LAYERS OF IRON AND NICKEL ALUMINIDES ON STEEL  Milena Voděrová 1,*, Pavel Novák 1  1 Department of Metals and Corrosion Engineering, ICT Prague, Technická 5, 166 28 Prague, Czech Republic  * corresponding author: tel. +420 220 445 026, e-mail: voderovm@vscht.cz   Resume Intermediary  phases  Ni-Al  and  Fe-Al  are  promising  materials  due  to  their superior properties such as hardness and good resistance  against oxidation at high temperatures. Moreover, Fe-Al phases are resistant in sulphur - containing atmospheres. Because of these characteristics, the above mentioned intermetallic phases seem to be prospective for the use in many technical applications such as energetics,  chemical  or  automotive  industry  in  a  form  of  a  bulk  material  or coatings. Presently, the protective aluminide layer is usually prepared by thermal spraying.  Nevertheless,  this  method  is  not  suitable  for  complex-shaped components. Therefore, the aim of this work was to find an alternative way to prepare layers consisting of nickel or iron aluminides by other technique than thermal spraying. At first, carbon steel samples were coated using galvanic or electroless  nickel  plating.  Coated  samples  were  subsequently  submerged  into molten  aluminium  at  various  temperatures  and  process  durations.  The influence of the temperature and duration on the intermetallic phase growth was  studied  by  scanning  electron  and  light  microscopy.  Thickness  and microhardness of the intermetallic layer was also measured.  Available online: http://fstroj.uniza.sk/journal-mi/PDF/2013/15-2013.pdf Article info  Article history: Received 17 December 2012 Accepted 30 April 2013 Online 28 July 2013    Keywords: Aluminide; Protective layer; Nickel coating; Fe2Al5.   ISSN 1335-0803 (print version) ISSN 1338-6174 (online version)  1. Introduction In  recent  years,  intermetallics  based  on aluminium and transition metals, e.g. nickel or iron,  have  been  widely  studied.  Generally, aluminides seem to be interesting materials in   a  bulk  form  or  coating  due  to  their  superior properties, such as low densities, high melting points and corrosion resistance. The strength of some  intermetallics  remains  satisfactory  with increasing temperature or can even increase as the temperature increases. Mentioned properties determine aluminides to be used especially for high-  temperature  applications.  Intermetallic phases  containing  Ni  and  Al  can  differ  from  Fe-Al in some points. The major advantages of nickel  aluminides  are  e.g.  resistance  to oxidation  and  carburization  in  both  oxidizing and  reducing  atmospheres  up  to  1100  °C, fatigue  and  wear  resistance  or  superior  creep strength.  [1,  2]  Iron  aluminides  are  promising while used in sulphur containing atmospheres, have  high  electric  resistivity  increasing  with temperature and remain stable in many aqueous solutions.  Although  it  seems  that  aluminides have only advantages, their ductility is limited because  of  high  hardness  and  brittleness. Manufacturing and processing of intermetallics is also limited. As it was mentioned above, due to  the  suitable  properties  of  aluminides protective layers formed by Al and Ni or Fe are perspective  e.g.  for  components  which  are exposed  to  elevated  temperatures  or  exhaust gases. There are many methods how to prepare aluminide  layer,  for  example  by  thermal spraying [3, 4], CVD [5-7] or by submersion of the  component  into  molten  aluminium.  Pre- plating of the substrate material, mostly steel, is usually realized by galvanic Ni plating [8, 9]. Within  galvanic  nickel  plating  which  is commonly used, electroless nickel coating was This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA. This copy of the article was downloaded from http://www.mateng.sk , online version of Materials Engineering - Materiálové inžinierstvo (MEMI) journal, ISSN 1335-0803  (print version), ISSN 1338-6174 (online version). Online version of the journal is supported by www.websupport.sk  . M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 113 used  in  this  work  to  prepare  homogeneous nickel  coating.  The  main  advantage  of  this process is that even complex-shaped materials can be coated with nickel of similar thickness on  all  the  parts  of  the  component.  The  only problem  is  ecological  disposal  of  waste solution. Aim  of  this  work  was  to  find an alternative  way  how  to  prepare  mentioned layers economically and simply with accent on using  it  on  complex  machine  parts  and elements.  Replacing  thermal  spraying  process by  submersion  the  sample  into  molten  metal seems to be perspective. 2. Experimental As a substrate material, plain carbon steel was  used  in  this  experiment.  Steel  contained 0.13 % C, 0.27 % Mn, 0.02 % S. Composition of  steel  was  determined  by  glow  discharge atomic emission spectrometry (GDS). Surface  of  rectangle  samples (2.5 x 2 x 0.2 cm) was treated by grinding and rinsed  by  water  and  ethanol  to  remove  iron oxides  and  impurities.  These  samples  were consequently  coated  with  nickel  by  two different  procedures  -  galvanic  nickel  plating and  electroless  nickel  plating.  Required thickness of nickel layer was approx. 10 μm.  2.1. Galvanic nickel plating Galvanic nickel plating was carried out in nickel plating solution which consisted of 60 g/l NiSO4,  120  g/l  Na2SO4  and  H2SO4.  The  pH value  of  the  bath  was  maintained  at  pH  =  3. Nickel  plating  was  carried  out  at  60  °C  with current  density  50  A/m2  and  voltage  between two lead anodes and steel cathode 4.5 V. Time of electrolysis was calculated to 10 minutes.  2.2. Electroless nickel plating This method is based on charge exchange reaction in water solution. Nickel is cemented on iron together with a small part of phosphorus (=  reduction)  and  hypophosphite  changes  into phosphate (= oxidation). Before plating in this nickel  bath,  the  samples  were  pickled  for 3 minutes in HCl 3 : 1 (acid : water) and then three minutes in H2SO4 1 : 3, cleaned by ethanol and dried by hot air. Electroless nickel plating proceeded  in  nickel  bath  which  consisted  of 34 g/l Ni(C3H5O3)2, 31,5 g/l Ni(H2PO2)2, 10 ml/l C3H6O3 and 5ml of NaOH per every 25 ml of solution.  Dissolving  the  content  was  taken  at 75 °C  and  coating  process  was  carried  out  at 90 °C for 120 minutes. After 40 and 80 minutes from  start  of  the  procedure,  NaOH  in  required amount was put into the bath. Coating prepared by electroless  plating  procedure  contained  17 at.% phosphorus which was determined by GDS.  2.3. Reaction synthesis of aluminides Coated  samples  were  submerged  into molten aluminium under normal pressure at 700 and  900  °C  for  various  time  periods  (30  and 240 s).  As  the  reference  materials,  two  steel samples without nickel coating were introduced to  the  aluminium  melt  under  the  same conditions. As  a  different  variant  of  the  aluminide synthesis,  one  sample  coated  by  galvanic process  was  put  into  aluminium  powder  and annealed at 650 °C for 8 hours.  2.4. Samples preparation and measurement After  the  coating  process  had  finished, samples  were  air-cooled  and  metallographic samples  were  prepared  as  perpendicular  cuts across  the  layer.  Samples  were  ground  using P60- P4000 abrasive papers and polished by D2 diamond  paste.  These  samples  were  observed by light and electron microscope.  Thickness  of  produced  aluminide  layer was  measured  by  image  analysis  of  the  light micrograph  using  LUCIA  4.8  image  analysis software.  Microhardness  (HV  0.05)  of aluminide layer was measured by Hannemann microhardness tester. Chemical  composition  of  aluminides  in the  layer  was  determined  by  Tescan  Vega  3 scanning  electron  microscope  using  Oxford Instruments  INCA  350  EDS  analyser  and compared with results obtained in [8]. M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 114 3. Results and discussion 3.1. Microstructure of aluminide layers In  Fig.  1,  diffusion  process  during submersion in molten aluminium is described. Diffusion of aluminium and iron is described by arrows.  Text  labels  mark  presence  of  phases formed in the early state of submersion. Molten aluminium enters the surface of nickel forming nickel aluminide Ni3Al and then iron. From the surface  inwards,  iron  diffuses  into  nickel  and aluminium  forming  solid  solution  Fe-Ni  and different types of iron aluminides- FeAl2, FeAl3, Fe2Al5.  Iron  aluminide  formation  is  probably     a  consequence  of  the  delamination  of  the produced nickel aluminide layer. At  first,  samples  produced  by                   a combination of galvanic nickel plating and hot dip  aluminizing  (Ni  GAL)  were  investigated. Electron  micrographs  show  the  representation of various intermetallic phases which originate from reactions during submersion. In the case of the sample submerged for 30 seconds (Fig. 2) there  still  remain  small  parts  of  non-reacted nickel. Second nickel-containing phase is NiAl3 aluminide.  Aluminium  diffuses  towards  the steel  substrate  and  forms  Fe2Al5  as  it  was described in [10]. This phase can be found in all the samples submerged for 4 minutes at 700 and 900 °C (Fig. 3 and 4). The difference is in the amount  of  nickel.  There  is  no  nickel  in  the elemental  form  or  in  nickel  aluminide.  Only small content of nickel (approx. 1 wt. %) was indentified  in  iron  aluminide.  Sample submerged  at  900  °C  contained  areas  of aluminide FeAl3, as seen in the picture. As  well as in  the  case  of  samples  with galvanic  coating  submerged  for  4  minutes, layers  produced  by  a  combination  of  the electroless nickel plating and aluminizing in the melt  (Ni-P)  exhibit  only  very  low  content  of nickel  in  the  intermetallic  layer  (Fig.  5,  6). Transition  zone  between  iron  and  aluminium consists of aluminide Fe2Al5, sample treated at 900 °C contained small areas of FeAl2 included in Fe2Al5.  Low nickel content in prepared layers can be  explained  as  following:  Reaction  which forms  aluminides  of  nickel  is  strongly exothermic.  Due  to  a  different  thermal expansion  coefficient  of  nickel  aluminide  and steel, the Ni-Al layer delaminates and sinks to the bottom of the melting crucible due to higher density than aluminium.    Fig. 1. Diffusion process during submersion.    Fig. 2. Ni GAL 700 °C / 0.5 min. M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 115  Fig. 3. Ni GAL 700 °C / 4 min.   Fig. 5. Ni-P 700 °C / 4 min.    Fig. 4. Ni GAL 900 °C / 4 min.   Fig. 6. Ni-P 900 °C / 4 min.  Due to extreme heat generation of Ni + Al reaction,  nickel  aluminide  can  be  even  molten and  dissolved  in  the  melt.  As  longer  is  the submerging  time  that  less  nickel  remains  on steel. Heat produced by the Ni+Al reaction then supports  the  formation  of  iron  aluminides. Samples without nickel coating were submerged at  700  °C.  Layer  originated  on  sample submerged  for  0.5  minute  consists  of  a  thin Fe2Al5  phase.  Sample  treated  for  longer  time period included layer of Fe2Al5, about 50 μm of thickness.  Third  technology  –  combination  of galvanic  plating  and  powder  aluminizing  – produced layers of Fe2Al5 of different thickness which  may  depend  on  amount  of  powder present on surface of steel.  3.2. Thickness and microhardness of aluminide layers Light  micrographs  reveal  that  the thickness  of  the  Fe2Al5  layer  increases  with process duration and temperature.  M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 116  Fig. 7. Fe 700 °C / 0.5 min.   Fig.8. Fe 700 °C / 4 min.  Fig. 9. Fe2Al5 made with Al powder.  An increase of the process temperature improves  the  diffusion  of  aluminium  into substrate  material  is  much  faster  and  as            a  result,  the  penetration  depth  is  higher. Thickness  of  the  layer  used  in  graphs  was created  as  the  mean  value  of  30  values measured  at  different  places  on  the  sample (Fig. 10). Presented  microhardness  HV  0.05 (Fig. 11) was averaged from 10 measurements. It could be seen that layers prepared at 900 °C reach  higher  values  of  hardness.  It  could  be explained  so,  that  formed  aluminide  of  iron Fe2Al5 can exist in wider range of Al and Fe concentration.  Ratio  of  iron  and  aluminium forming  Fe2Al5  can  differ  a  little  bit  from aluminide originating at 700 °C. If there is little more iron included, microhardness can increase. Influence  of  phosphorus  in  electroless  nickel coating is discussed.  M. Voděrová, P. Novák: Protective layers of iron and nickel aluminides on steel    Materials Engineering - Materiálové inžinierstvo 20 (2013) 112-118 117  Fig. 10. Thickness of Fe2Al5.   Fig. 11. Microhardness of Fe2Al5.  4. Conclusions Introduced  methods  are  not  suitable  for producing compact layers containing aluminides of nickel. Only in the case of sample submerged for  0.5  minute  there  was  a  small  amount  of NiAl3 and nickel in elemental form. Because of intensive  exothermic  reaction,  originating aluminides of nickel delaminate from steel and dissolve  in  molten  aluminium.  On  the  other hand, these methods are promising for creating Fe-Al layers. It was demonstrated that there is an  influence  of  temperature  and  submersion duration  on  thickness  and  microhardness  of layer. Samples submerged for the same time at higher temperature are characterized by thicker layer  composed  mainly  of  Fe2Al5  with  small parts  of  FeAl2  or  FeAl3.  Thickness  of  layer varies  in  range  of  30-  200  μm.  There  is  also       a  difference  between  microhardness  of  layers produced  on  galvanic  and  electroless  nickel coatings.  Layers  produced  at  higher temperatures  are  harder  because  of  different aluminium- iron ratio in Fe2Al5. There may be an influence of phosphorus present in the case of electroless nickel coatings, which may form hard  phosphides.  Presence  of  nickel  coating may influence kinetics the iron aluminide layer formation.  Powder  aluminizing  produced  the layer formed by Fe2Al5 as in the case of melt submersion process, but the laik, yers are less homogeneous.  Acknowledgment This research  was financially  supported by  Czech  Science  Foundation,  project  No. P108/12/G043 and by Grant Agency of ASCR, project No. KAN300100801. M. Voděrová, P. 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PROTECTIVE LAYERS OF IRON AND NICKEL ALUMINIDES ON STEEL

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