Browsing by Author "Pehkonen, Antero"

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  • Passivoituvien metallien aktivointi kovakromauksessa
    (2009) Joas, Tony
     Helsinki University of Technology | Master's thesis
    The aim of this thesis was to find and examine existing activation methods for passive metals in hard chromium plating. Another target was to investigate, which factors have an effect on successful activation process, and to discover a method to measure the activation process with electrochemical measurement methods. In the theoretical part of this work, the principals of hard chromium plating, such as chromium deposition, structure of deposited coating and pre-treatment methods are introduced. The activation process is examined more closely. Activation methods that have been found effective are processed and listed. This research focused on activation methods that are suitable for AISI 316, Monel 400, nickel and Nimonic 75. The preferred activation method for passive metals in literature was cathodic activation in sulphuric acid solution or in sulphuric and hydrofluoric acid solution. In the experimental part of the thesis, suitable activation methods and parameters for passive metals in chromium bath and 10 vol-% sulphuric acid were studied using electrochemical methods. Research methods were potential measurement, polarization, potentiostatic method and impedance measurement. The best surface cleanliness was achieved by using 10 min cathodic activation in sulphuric acid. AISI 316 was held in potential -500 mV, Monel 400 in potential -450 mV, nickel in potential -400 mV and Nimonic 75 in potential -500 mV. Based on the laboratory experiments, metals become passive very quickly after cathodic activation. It was assumed that the time between activation and chromium plating should be as short as possible. The rate of passivation can be easily measured by potential measurement. The influence of passivation and the use of potential measurement in activation control were studied in factory conditions. The plant experiments showed that surface pre-treatments and activation methods seem to have more influence in successful chromium plating than passivation of the surface.
  • Artificial patination of architectural copper
    (2007) Korpinen, Tapio Oiva
     Helsinki University of Technology | Licentiate thesis
  • Comparison of corrosion properties of chromate free passivation treatments
    (2011) Lepikko, Emmi
     School of Chemical Engineering | Master's thesis
  • Corrosion of copper by water under oxygen-free conditions
    (2010) Ikonen, Johannes
     School of Chemical Engineering | Master's thesis
  • Sellutehtaan rakennemateriaalien korroosionkestävyys peroksohappojen valmistuksessa ja käytössä
    (1995) Hildén, Jouko
     Helsinki University of Technology | Master's thesis
  • Eräiden kupariseosten korroosionkesto muurahaishappoliuoksissa
    (2005) Leppänen, Hanna
     Helsinki University of Technology | Master's thesis
    The purpose of this thesis was to find the best corrosion resistant copper alloy for copper tubes, which are used in heat-exchanging, refrigerating and air-.conditioning units. In theoretical section of the thesis corrosion mechanism, in-Influencing factors, history and cases of formicary corrosion were studied. In same context also widely used method in reproduction test and different preventive measures based on literature survey were presented. In addition of this, I corrosion of aluminium and zinc in formic acid solution were also studied. In experimental part of the thesis the corrosion behaviour of pure copper, aluminium, tin, zinc and some copper alloys were studied in formic acid solutions. The behaviour of pure copper was also studied in sulphuric acid solutions. Linear polarization resistance, anodic and cathodic polarization and impedance measurements were used as research methods. Several electrolytic solutions were used in the investigation. In preliminary study the effect of three different formic acid solutions on pure copper was investigated. Based on this preliminary study aqueous solution containing 5 vol % formic acid was chosen and in that solution all the other materials were studied. Due to comparison the effect of pure copper was studied in two sulphuric acid solutions, which had the same pH-values as equivalent formic acid solutions. Different materials were also studied in formic acid solution whose pH-value was raised and in a solution where copper ions had added to that. In addition test similar to the reproduction test, was carried out on the copper tubes. The purpose of that test was to repeat the test and compare the results with results of other researchers. Based on tests it was concluded that pure aluminium was the least corroding metal in test conditions. However, no big differences were found in the behaviour of the different copper alloys. Copper alloy no. 12 and Nordic gold were least corroding metals in the group of copper alloys. Formicary corrosion was not noticed in reproduction test. If there were formicary corrosion on copper tubes it was very difficult to find. Based on test formicary corrosion does not seem to be an own corrosion.
  • The effect of dissolved ozone on the corrosion behaviour of some stainless steels
    (2001-12-14) Pehkonen, Antero
     Doctoral dissertation (monograph)
    The corrosion behaviour of some stainless steels and pure metals has been investigated in solutions with dissolved ozone. The pH of the test solution was 1, 2, 3 and neutral (adjusted by H2SO4). 2.3 and 23 g/l Na2SO4 was added to increase the conductivity in anodic and cathodic polarisation measurements. The temperature of the test solution was 20, 50 and 75 °C. Test materials were four different stainless steels: ferritic stainless steel Polarit 815 and austenitic stainless steels Polarit 720, Polarit 752 and Ralloy 654MO. Also Armco iron, pure Mo, Ni and Cr were tested. The general electrochemical behaviour of stainless steels and pure metals was studied with potentiodynamic cyclic polarisation experiments at scanning rates 1, 10 and 100 mV/min and pitting behaviour at 10 mV/min. Potentiostatic experiments were also used. Immersion tests were also carried out mainly to produce test samples for surface examination but also to measure the weight losses. The structure and thickness of the oxidized surfaces were examined and analysed using optical microscope, SEM, GDOS, ESCA and X-ray diffractometer methods. Dissolved ozone increases the redox-potential of test solutions to about +1200 mV vs. SCE and the corrosion potential of stainless steel to the transpassive region, below the oxygen evolution potential. The current densities of stainless steels in this region increase as the amount of alloying elements increases. On the other hand, dissolved ozone increases current densities only slightly compared to oxygen bubbled solution. Higher ozone concentration obtained under high pressure has no effect on the corrosion behaviour of stainless steels. The corrosion of stainless steels is general but peeling of oxide layers was also observed. Shallow corrosion damages are formed in immersion. In cyclic polarisation experiments pits are formed in Ralloy 654MO. The pits formed are similar to those formed by chloride induced pitting. As the pH of the solution is decreased, the current density of higher alloyed steels increases at higher pH than lower alloyed steels. Dissolved ozone increases the thickness of the oxide layers greatly. In oxygen bubbled solution at pH 3 the thickness of the layers is a few nanometers and in ozonated solution 60 - 260 nm. The compositions of these oxide layers has been found to vary greatly. Cr concentration is negligible in the oxide layers especially for Ralloy 654MO and P752. Molybdenum concentrates in the outer layer, even more so with lower alloyed steels. Iron and alloying elements have oxidised to their highest valencies. The main components in the oxide layers are iron oxides and on the surfaces also MoO6 occurs.
  • Electrochemical determination of hydrogen entry to HSLA steel during pickling
    (2018-01-01) Aromaa, Jari; Pehkonen, Antero; Schmachtel, Sönke; Galfi, Istvan; Forsén, Olof
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Pickling with hydrochloric acid is a standard method to clean steel surfaces before hot-dip galvanizing. When normal low strength steels are pickled, hydrogen formed in pickling reactions does not have any significant harmful effect on the mechanical properties of steel. However, in pickling of steels with higher strength, the penetration of hydrogen into the steel may cause severe damages. The effect of pickling of high-strength low-alloy (HSLA) steels was investigated using a cell construction based on the Devanathan-Stachurski method with modified anodic surface treatment and hydrogen production using acid. The penetration and the permeability of hydrogen were measured using an electrochemical cell with hydrochloric acid on the one side of the steel sample and a solution of NaOH on the other side. No protective coating, for example, palladium on the anodic side of the sample, is needed. The penetration rate of hydrogen into the steel and exit rate from the steel were lower for higher strength steel.
  • Henkilöauton pakoputken korroosio
    (2011) Domnina, Jekaterina
     Insinööritieteiden korkeakoulu | Bachelor's thesis
  • Hydrometallurgiset menetelmät elektroniikkaromun metallien talteenotossa
    (2011) Heikkilä, Juho Oskari
     Kemian tekniikan korkeakoulu | Bachelor's thesis
  • Hydrometallurgiset metallien talteenottomenetelmät paristoromusta
    (2011) Rouvinen, Jatta-Mari
     Kemian tekniikan korkeakoulu | Bachelor's thesis
  • Korroosio elektroniikan elinkaaressa
    (2010) Henriksson, Tuomas
     Kemian ja materiaalitieteiden tiedekunta | Bachelor's thesis
  • Kovakromipinnoitus ja sen korroosiokestävyys
    (2009) Lepikko, Emmi
     Kemian ja materiaalitieteiden tiedekunta | Bachelor's thesis
  • Messingin jännityskorroosio
    (2012-11-30) Pylväläinen, Heini
     Kemiantekniikan korkeakoulu | Bachelor's thesis
  • Monitoring of reinforcement corrosion by use of the galvanostatic pulse method
    (2004) Tiirikainen, Suvi
     Helsinki University of Technology | Master's thesis
    Interpretation of corrosion risk in reinforced concrete structures has been performed with a number of electrochemical techniques ever since the first half-cell potential mapping measurements were developed. The maintenance and repair of the structures requires a monitoring technique that can reliably locate the corroding areas and estimate corrosion rates. The galvanostatic pulse technique introduced in 1988 has been lately investigated as an easy, rapid and non-destructive method to assess corrosion rate of rebar steel. The aim of the study was to determine the usability of galvanostatic pulse measurement applications on corrosion monitoring of reinforced steel in concrete to assess corrosion rates. The laboratory tests were performed in simulated pore water solutions with varying chloride concentrations. The purpose of tests was to define parameters, which give the most reliable and comprehensive information of the condition of rebar steel rods. The traditional electrochemical methods as well as the galvanostatic pulse method were used to specify parameters in laboratory and they were tested using the GPM method on site. The main interest was to examine reinforced concrete structures guarded by cathodic protection. The tested galvanostatic pulse method was proved to be suitable to separate active and passive state of steel. The measurements performed in laboratory confirmed that there is one decade difference in corrosion current density values between active and passive steel. The results showed that the magnitude of the cathodic protection current and the applied measurement current do not have an excessive influence on the determined resistances. However the determination of resistance values may be disturbed related to the small applied currents if corrosion rate is high and deviations occur in the measured transient. Therefore the interrupted GPM method was more applicable because the time constants could also be used to determine the corrosion state of steel.
  • Recovery of metals in mobile phones
    (2009) Karonen, Mari
     Helsinki University of Technology | Master's thesis
    Current manufacturing of mobile phones uses mainly natural ores as the resource for the metals needed. Recycling and recovery of metals is an important area to focus on to reassure the sufficiency of the recourses in the world in the future. Nowadays the recycling of mobile phones starts with disassembly of the battery. Then 1) Phones can be crushed, when the subsequent steps are separation of metals from plastics and pyro metallurgical thermal or hydrometallurgical processes. Or 2) The Printed Wiring Board, display and covers can be dismantled and channelled to different metallurgical recovery processes. The majority of the metals in a phone are copper therefore the processes are based on the pyro metallurgical production of copper. The valuable metals, whose amounts are small compared to copper, are precipitated after copper in the hydrometallurgical processes such as leaching to aqua regia or cyanide solutions. The purpose of the thesis has been to explore the different methods for the recycling of mobile phones and recovery of metals from the view point of a metallurgist. The emphasis has been on hydrometallurgical methods and seeking a simple process to leach all the metals at the same time. The subsequent recovery of metals, which has not been under further study, is done by precipitation from the solution one by one or all at the same time. The application of the HydroCopper® process by Outotec to leach printed wiring boards from old mobile phones was studied in the experiment part of the thesis. Ozone is known to react strongly with organic materials; therefore ozone flow has been experimented to intensify the separation of plastics and metals. Based on the experiments can be claimed that the HydroCopper solution is strong enough to leach nearly every metal included in a mobile phone within a reasonable time. The effect of ozone was not as strong as assumed. It increases only slightly the dissolution of the plastic material in PWBs.
  • Ruostumattomien terästen elektrolyyttinen kiillotus
    (2009) Salmi, Kim Kristian
     Kemian ja materiaalitieteiden tiedekunta | Bachelor's thesis
  • Ruostumattomien terästen värjäys
    (2008) Lehmusmies, Simo
     Kemian ja materiaalitieteiden tiedekunta | Bachelor's thesis