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Author:Joas, Tony
Title:Passivoituvien metallien aktivointi kovakromauksessa
Activation of passive metals in hard chromium plating
Publication type:Master's thesis
Publication year:2009
Pages:98      Language:   fin
Department/School:Materiaalitekniikan laitos
Main subject:Korroosio ja hydrometallurgia   (MT-85)
Supervisor:Forsén, Olof
Instructor:Pehkonen, Antero
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark TKK  113   | Archive
Keywords:hard chromium plating
activation
kovakromaus
aktivointi
Abstract (eng): 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.
Abstract (fin): Tässä työssä on tutkittu passivoituvien metallien kromausta edeltävää aktivointia.
Tavoitteena oli selvittää tekijät, jotka vaikuttavat aktivoinnin onnistumiseen.
Samalla selvitettiin mahdollisuutta aktivoinnin mittaamiseen sähkökemiallisilla mittausmenetelmillä.

Työn teoriaosassa käsiteltiin kovakromauksen perusteita, kuten kromin saostumista, pinnoitteiden rakennetta ja kromattavan metallin esikäsittelymenetelmiä.
Erityisesti haettiin tietoa metallipinnan aktivoinnista ennen kovakromausta ja nimenomaan erilaisia käytössä olevia ja tehokkaita aktivointimenetelmiä.
Aktivointimenetelmissä keskityttiin erityisesti työssä tutkittaviin metalleihin, joita olivat ruostumaton teräs AISI 316, nikkelikupariseos Monel 400, puhdas nikkeli, sekä nikkelikromiseos Nimonic 75.
Passivoituville metalleille suositeltiin kirjallisuuden perusteella katodista aktivointia rikkihappopitoisessa liuoksessa, johon on lisätty hieman fluorivetyhappoa.

Kokeellisessa osassa tutkittiin sähkökemiallisilla tutkimusmenetelmillä mahdollisia aktivointimenetelmiä ja -parametreja koemateriaaleille sekä kromikylvyssä että 10 til-% rikkihappoliuoksessa.
Tutkimusmenetelminä käytettiin polarisaatioajoa, potentiaaliseurantaa, potentiostaattisia mittauksia ja impedanssimittauksia.
Rikkihappoliuoksessa suoritetuissa kokeissa puhtain pinta saatiin 10 min katodisella aktivoinnilla.
Materiaalille AISI 316 suoritettiin aktivointi potentiaalissa -500 mV, Monelille potentiaalissa -450 mV, nikkelille potentiaalissa -400 mV ja Nimonicille potentiaalissa -500 mV.
Aktivoinnin mittaaminen sähkökemiallisilla menetelmillä osoittautui hankalaksi.
Laboratoriokokeissa havaittiin koemateriaalien passivoituvan erittäin nopeasti katodisen aktivoinnin jälkeen.
Sen perusteella oletettiin, että aktivoinnin ja kromauksen välisen ajan tulee olla mahdollisimman lyhyt.
Metallien passivoitumista pystytään seuraamaan potentiaaliseurannalla.
Tehdaskokeilla selvitettiin passivoitumisen merkitystä ja potentiaalimittauksen käytettävyyttä aktivoinnin mittauksessa.
Kokeiden perusteella pinnan esikäsittelyllä ja aktivointimenetelmillä vaikuttaisi olevan enemmän merkitystä kromauksen onnistumiseen kuin pinnan passivoitumisella.
ED:2012-08-31
INSSI record number: 45170
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