Metal yüzeylerde oksit tabakası oluşumunda polimer malzemelerin etkisinin incelenmesi
Investigation of the effect of the polymeric materials on the anodic oxidation on the metal surfaces
Yazar: AZİZE GÖMLEKSİZ
Danışman: DOÇ. DR. ALİ GELİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Fizik Mühendisliği Ana Bilim Dalı / Fizik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Alüminyum sanayisinde metal yüzeyinin korozyon direncini arttırmak, alüminyum yüzeyini boyayabilmek veya dekoratif bir görünüm elde edebilmek amacıyla gerçekleştirilen anodizasyon işlemi büyük tanklarda tonlarca asit çözeltisi kullanılarak yapılmaktadır. Anodizasyon işlemi sırasında açığa çıkan asit buharının operatörlerin sağlığına olan olumsuz etkisi, sonrasında ise kullanılan tonlarca asit atığının doğaya verdiği zararın boyutları daha çevreci bir yöntemin varolup olmadığı sorusunun önemini attırmaktadır. Ayrıca kullanılan geleneksel yöntemdeki kontrolsüz iyon göçü işlemin daha uzun sürelerde tamamlanmasına neden olup enerji verimi açısından dezavantajlıdır. Alüminyumun büyük asit tankları yerine yüzeyine, cüzi miktarda asit çözeltisi içeren jel-elektrolit kontak edilerek eloksallanması doğaya ve işlemi yürüten operatörlere verilen zararı büyük ölçüde minimize edecektir. Bu motivasyonla gerçekleştirilen bilimsel çalışmada amaç jel tabanlı anodizasyon hücresi geliştirmek ve üretilen
alüminyum oksit tabakalarının özelliklerini araştırmak olmuştur. Poliakriamid jeller sülfürik asit çözeltisinde şişirilip alüminyum yüzeyine kontak edilerek üzerinden elektrik akımı geçirilmiştir. Uygulanan potansiyel gerilim ile iyonlarına ayrılan asitteki ve sudaki oksit içeren anyonlar elektrik alan kuvvetiyle, hücreye anot olarak kutuplanmış alüminyumun yüzeyine sürülür ve elektron kaybetmiş alüminyumla reaksiyona girip alüminyum oksit tabakası oluşturur. Elektroliz hücresinin tasarımında jel-alüminyum kontağını düzgün ve homojen biçimde presleyebilecek kapalı bir sistem tercih edilmiştir. Oksit tabakasının oluşumuna etki eden parametreler üzerinde çalışılarak üretilen alüminyum oksitin özellikleri yorumlanmıştır. Bu parametreler, anodizasyon hücresine uygulanan gerilim, elektroliti hapsedecek polimerin konsantrasyonu, anodizasyon süresi ve jelalüminyum kontağına uygulanan basınçtır. Deneyler boyunca elektroliz hücresinden geçen akımın zamana göre davranışı kaydedilerek gözlemlenmiştir. Yapılan çalışmalar sonucunda, en kaliteli alüminyum oksit tabakasının üretildiği optimize
değerler elektroliz hücresine 30V'luk potansiyel gerilimin uygulandığı ve 4M'lık jel elektrolitlerin kullanıldığı numuneler olarak belirlenmiştir. Anodizasyon süresinin parametre olarak alındığı deneylerde ise, 15 saniye gibi kısa bir sürede dahi düzgün altıgen yapılı nanogözenekli oksit tabakası oluşturulabilmesi jel tabanlı anodizasyon
metodunun enerji verimi açısından ne denli avantajlı olduğunun açık göstergesidir. Ayrıca geleneksel yöntemin aksine elektrolitin jel içerisinde tuzaklanması alümiyumun lokal olarak eloksallanmasına da imkan vermektedir. Bu özellik sayesinde desenli renkli dekoratif alüminyum yüzeyleri elde edilebilecektir. Jel elektrolit ile eloksallama yöntemi oksit tabaka üzerinde bıraktığı polimer kirliliği gibi çözülmesi gereken sorunları beraberinde getirse de geleneksel anodizasyon yöntemine alternatif olabilecek çevre dostu ve enerji tasarrufu açısından verimli bir metottur.

Abstract:
Anodization is a method to develop anodic oxide films on a metals surface in order to protect the metal from further oxidation in harsh conditions such as high temperature and humidity. The name anodization comes from connecting the metal as an anode to the electrolytic cell. By this treatment the thin natural oxide layer formed on the surface of the aluminum due to the reaction with air can be thickened. This aluminum oxide layer is a compact solid structure that protects the aluminum, unlike iron oxide where the rust of the iron comes off in scales. An anodized aluminum has the advantageous of being more resistant to corrosion, having a dyeable porous surface and a cosmetic finish. It has also different application areas such as supercapacitor production and icephobic surface preparation. The conventional anodization method is based on applying a constant voltage
difference to the electrodes of the electrolytic cell, where aluminum is connected as an anode. Due to this applied voltage the aluminum is forced to lose electrons and become Al3 cation, and the acid solution in water which is preffered as an electrolyte, breaks up into its ions. The oxygen containing anions is forced to travel towards the surface of the aluminum which is connected as an anode to the cell, and get into reaction with
the Al3 cations and create an aluminum oxide Al2O3 layer at the surface of the metal. This electrochemically produced aluminum oxide layer is selectively dissolved by the acid solution, and if the oxide formation rate is bigger than the oxide dissolution rate, self arranged hexagonal nano pourous Al2O3 is obtained at the surface of the aluminum. Applying constant voltage difference to the cell is called as potentiostatic method. When aluminum is anodized with potentiostatic method, by the time, the current flowing through the system will decrease and will minimize to the leakage current which leaks due to the structural defects of the aluminum. While current
decreases the thickness of the oxide layer grows. This happens because the Al3 cations cannot travel across the thickened oxide layer in order to react with O-2 and form Al2O3, in the same way O-2 also cannot pass the oxide layer and get into reaction with Al+3 cations. Therefore in anodization process, after a certain thickness of oxide layer the current passing through the system is limited due to the delimited ion migration and further oxidation cannot be done. Thanks to anodization method two type of oxide layer can be obtained depending on the type of eletrolyte used in the cell. One of them is the barrier type oxide layer which occurs in the presence of neutral or mild alkaline electrolyte and the other one is the nanoporous oxide layer which is formed when acid
solution is used as the electrolyte. The aluminum industry is mainly focused on producing a porous oxide layer because the rough surface gives opportunity for dying or having various types of decorative finishes. In this scientific research the main focus is also producing porous oxide layers but with a gel-based anodization system instead of the conventional one, in order to investigate the feasibility of the novel method to determine whether it has the potential to be an alternative or not. The conventional anodization of aluminum takes place in big acid tanks where huge amount of acid is used as electrolyte. In the conventional method, the hazardous effect given to the operators health by the released acid vapor during the anodization process, and after the anodization process, the harm given to the environment by the waste of huge amount of used acid, give rise to thought whether there is an environmentally friendly method or not. Additionally, the uncontrolled complex ion migration in the
conventional method due to the large reservoir of electrolyte increases the anodization duration which causes high energy consumption. Furthermore, the non uniform electric field created in the acid tank results in obtaining nonhomogeneous surface of oxide layer on the aluminum. Instead of anodizing the aluminum in acid tanks, using
a gel electrolyte-aluminum contact to produce anodic oxide layer at the surface of the aluminum will minimze the harm given to the health of the operators and to the environment, because of the small amount of acid confined by this gel electrolyte, where the waste and the vapor of the acid are tolerably few. Also, the anodization
duration will be reasonably minimized in the gel based anodization system compared to conventional method because the ion migration is not sophisticated at the fairly small reservoir of the gel medium, which has nearly 3mm width that ions must travel across. In addition, thanks to the homogeneous electric field distribution due to the
geometry of the electrodes, uniform oxide layers having a smooth morphology can be produced in this novel method different than conventional anodization process. This research is done in order to develop a gel based electrochemical anodization cell and investigate the properties of the produced anodic oxide films via this novel
method, by the motivation of being able to anodize aluminum with an environmentally friendly method, which has therewithal high energy efficiency and capability to produce qualified oxide films in terms of morphology. Firstly, synthesized polyacrylamide gels at different concentrations which contain sulphuric acid solution
at distilled water were connected to the aluminum surface and then electric current was forced to pass through this gel-aluminum contact by applying voltage difference to the electrodes which also compress the gel-aluminum contact from both ends. Sulphuric acid and water is broken into its ions due to the applied voltage and the electric force drives the oxygen containing anions towards the surface of the aluminum which is connected as an anode to the electrolytic cell. When the oxygen containing anions meet with electron lost aluminum they will react and create an aluminum oxide layer on the surface of the aluminum. The gel based electrolytic cell was designed as a closed system (but having a gas outlet to send away the hydrogen gas occured as a reaction product) which compresses the gel-aluminum contact uniformly and homogeneously between two inert electrode inside a tube. The parameters which affect the anodic oxide formation was studied and the properties of the produced oxide layers were interpretted. Those parameters are the applied voltage to the electrolytic cell, concentration of the polymer gel (polyarcylamide) which confines the sulphuric acid eletrolyte, anodization duration, and the pressure that is applied on the gel-aluminum contact. During the anodization experiments the behaviour of the current flowing
through the electrolytic cell was observed and recorded via computer connected ammeter. As a result of the studies, optimized values that produced the highest quality aluminum oxide layer were determined by observing the SEM images of the samples. The optimized values are decided as 30V potential difference that was applied to the
electrolysis cell, and 4M polyacrylamide gel electrolytes which were used in the gelalu contact as a reservoir of ions that is needed to anodize the aluminum. In the experiments where the anodization time is taken as a parameter, the fact that a smooth hexagonal nanoporous oxide layer can be formed even in a short time range as 15
seconds, is a clear indication of the advantage of the gel-based anodization method in terms of energy efficiency. In addition, trapping of the electrolyte in the gel which brings a rigid structure to the elecrtolyte, in contrast to the conventional method, allows local anodizing of the aluminum. Thanks to this feature, colored decorative aluminum surfaces with certain patterns can be obtained for special applications such as colored company names or logos. Although the method of anodizing with gel electrolyte causes problems such as polymer pollution left on the oxide layer, it is an environmentally friendly and energy efficient method that can be an alternative to the
traditional anodization process. In order to prevent the polymer pollution left on the oxide layer, one of the important parameter on which attention must be paid is the drying rate of the gel which happens as a result of the excess heat occured due to the chemical reactions inside the electrolysis cell. Therefore, to be able to produce a
smooth surface of oxide, the oxide formation rate must be bigger than the drying rate of the gel, and the anodization duration must be kept small enough to complete the oxide formation before the gel gets dried. Additionaly, using loose gel which has low concentration, contributes to the pollution because the loose structure fits with the roughness of the surface of the aluminum well, and the gel aluminum contact merges undesirably. With those suggested improvements, the novel gel-based anodization system has a great potential to be an alternative method for anodization process with new features that does not exist in the conventional applications.