Elektrolitik çinko kaplamalı plakalarda hızlandırılmış korozyon testi uygulaması
Application of accelerated corrosion test for electrotic zinc coated plates
Yazar: ARİFE GÜLSÜM GÜLÇELİK
Danışman: DOÇ. DR. TURGUT GÜLMEZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Atmosferik korozyon = Atmospheric corrosion ; Çinko kaplama = Zinc coating

Özet:
Metal Kaplama günümüzde çelik yapıların yüzeylerini korozif ortamlara karşı korumak için kullanılan ticari açıdan önemli tekniklerden biridir. Metal Kaplama türlerine çinko, fosfat, krom, nikel, bakır ve bunların kombinasyonları örnek olarak verilebilir.
Metal Kaplama Yöntemleri ise Sıcak Daldırma, Sıcak Püskürtme ve Elektrolitik olarak sıralanır. Sıcak Daldırma Yönteminde, kaplama metali ergitilir ve ana metal bu ergiyik içerisine bırakılarak kaplama yapılır. İşlem esnasında ana metal yüzeyi, kaplama metali ile alaşımlı hale gelerek korozyondan korunmaktadır. Sıcak Püskürtme Yönteminde kaplama metali toz haline getirilir ve bir ısı kaynağı ile ergitilerek, ana metal üzerine püskürtülmesiyle kaplama yapılmaktadır. Bu işlem esnasında kaplama metali, ana metal üzerine yapışmaktadır. Sherardizing Kaplama Yönteminde, toz halindeki kaplama metali ile ana metal aynı kasa içerisinde konularak, yüksek sıcaklıkta bir müddet bekletilmektedir. Kaplama metali, sıcaklığın etkisiyle uçuculuk özelliği kazanmakta ve ana metali sarmaktadır. Elektroliz Yönteminde ise ana metal katot, kaplama metali de anot olacak şekilde, uygun bir elektrolit içerisinde kaplama yapılmaktadır. Bu işlem sırasında, kaplama metali korozyona uğrayarak ana metalin üzerini kaplar ve korozyondan korunmasını sağlar.
Galvaniz korozyona karşı metalin "Çinko" ile kaplanmasıdır. Galvaniz Kaplama 2 şekilde yapılır. Birincisi Elektro Galvaniz ile kaplama ikincisi Sıcak Daldırma Galvaniz Kaplamadır. Galvaniz kaplama, demir ya da çeliklere uygulanır. Metale çinko kaplanarak aşınma direnci arttırılır. Çinko demir ya da çelikten daha reaktif olduğu için, ilk olarak çinko galvaniz kaplama korozyona uğrayarak alt katmanda bulunan demir ya da çeliği korur.
Çinko kaplama bundan 200 yıl önce kullanıma girmiş ancak teknolojik açıdan bakıldığında, galvanizleme prensipleri değişmeden kalmıştır. Bununla birlikte, otomotiv ve inşaat sektöründeki yeni uygulamalar nedeniyle, galvanizleme sürecinin tüm yönleri ve yeni çinko kaplama türleri üzerinde önemli miktarda araştırma yapılmıştır. Bu gelişmelere paralel olarak kaplamalı malzemelerin korozyon dayanımları ve yaşlanması, malzemede zaman içerisinde oluşabilecek hasarın saptanabilmesi konuları da önem kazanmıştır. Bu anlamda malzemelerin korozyon dayanımlarının belirlenebilmesi için testler ve cihazlar tasarlanmıştır. Bu test ve cihazlar korozyon nedeniyle oluşan malzeme kusurlarının analizini ve seçilen malzemenin kalite güvencesini kontrol altında tutma imkanı verirken aynı zamanda yeni teknolojik malzemelerin, boya ve kaplama yöntemlerinin geliştirilmesine de girdi oluşturmaktadır.
Korozyon tanımı en genel şekliyle malzemelerin zaman içerisinde çevre etkisiyle bozularak kullanılmaz hale gelmesidir. Malzemenin ömrü korozyon direnci ile direk ilişkilidir. Malzemelerin servis ömrü konusunda bilgi sahibi olmak için korozyona uğrama süresini bilmek oldukça önemlidir. Malzemelerin korozyon dirençlerini saptayabilmek için dünya çapında farklı disiplinler tarafından oluşturulan standartlarda farklı kaplama tipleri ve kalınlıklarına göre korozyon testleri belirlenmiş ve bu testerin gerçekleştirileceği test düzenekleri tasarlanmışdır. Kaplama tipine ve kaplama kalınlığına göre söz konusu standartlarda belirtilen şartlarda yapılan testler günümüzde malzemenin korozyon direnci hakkında bilgi sahibi olabilmek için kullanılmaktadır. Korozyon için gerekli olan çevresel etkiler (nem, ısı, sıcaklık, tuzluluk) deneysel şartlarda kısmen sağlanabilirken zaman konusunda hala kısıtlar bulunmaktadır. Korozyon testlerinin çok uzun sürelerde tamamlanması söz konusudur. Kısa zamanda yapılan yüksek güvenilirlikteki testlerin ve cihazların geliştirilmesi için günümüzde hala hızlandırılmış korozyon testi üzerine çalışmalar devam etmektedir.
Bu çalışma da çinko kaplama tekniklerinden biri olan elektrolitik galvaniz kaplamalı plakalar üzerindeki kaplamanın korozyon dayanımı araştırılmıştır. Korozyonu katalize edecek olan etkiler araştırılarak yapılan araştırmalar doğrultusunda korozyonu hızlandıracak olan parametrelerin değiştirilmesi ile farklı deney koşullarında testler tasarlanmıştır. Bu bilgiler doğrultusunda tasarlanan farklı test düzeneklerinde 7 mikron kalınlıklarındaki çinko kaplamalı düz plakaların Tuz Sis Püskürtme Korozyon Testi yöntemiyle sıcaklık, tuz oranı ve pH parametrelerindeki değişkenliğin korozyon zamanına olan etkisi gözlemlenmiştir.
Zaman ve maliyet konusundaki kısıtlardan dolayı değişken parametrelerin her birinin minimum, maksimum ve orta noktaları kullanılarak deneyler gerçekleştirilmiştir. Deney tasarımı Minitab Paket Programı kullanılarak, DOE (Design of Expirement) yönetimi ile yapılmış olup elde edilen deney sonuçları aynı program kullanılarak analiz edilerek yorumlanmıştır. DOE Yöntemiyle minimum ve maksimum değerlerin aralıklarından yapılmayan deney şartlarındaki korozyon süreleri istatistiksel olarak belirli güvenilirlik değerleri ile elde edilmiştir ve program tarafından korozyon süreleri için formüller çıkarılmıştır.

Abstract:
Metal coatings on steel is one of the commercially most important processing techniques used to protect steel components exposed to corrosive environments. Zinc, phosphate, chromium, nickel, copper and their combinations can be given as examples for metal coating types.
Metal coating methods are Hot Dip, Hot Spray, Electrolytic and Sherardizing. In the Hot Dip Method, the coating metal is melted and the base metal is put into this melt and the coating is made. During the process, the base metal surface is alloyed with the coating metal and protected from corrosion. In the Hot Spray Method, the coating metal is powdered and melted with a heat source and coated by spraying on the base metal. During this process, the coating metal adheres to the base metal. In the Sherardizing Coating Method, the powdered metal and the base metal are placed in the same casing and kept at high temperature for a while. The coating metal gains volatility due to the temperature and wraps the main metal. In the Electrolysis Method, the main metal cathode and the plating metal are anode in an appropriate electrolyte. In this process, the coating metal is covered by corrosion and protects the base metal from corrosion.
Galvanized is coating process used by Zinc against the corrosion.Galvanized coating is applied in 2 ways. First is Electro Galvanizing and the second one is Hot Dip Galvanized Coating. Galvanized coating is applied to iron or steel. Abrasion resistance is increased by zinc coating on metal. Due to zinc is more reactive than iron or steel, the zinc galvanized coating first corrodes and protect the iron or steel that in the lower layer.
From a technological standpoint, the principles of galvanizing have remained unchanged since this coating came into use over 200 years ago. Zinc coating provides corrosion resistance to the steel fastener by acting as a barrier  and sacrificial coating. Zinc is more electrochemically reactive than steel, so when exposed to a corrosive environment, the zinc plating  corrodes sacrificially, delaying rust formation on the fastener even after portions of barel steel are exposed. However, because of new applications in the automotive and construction industry, a considerable amount of research has recently occurred on all aspects of the galvanizing process and on new types of Zn coatings. In parallel with these developments, the corrosion resistance and aging of the coated materials and the determination of the damage that may occur over time in the material have gained importance. In this sense, tests and devices are designed to determine the corrosion resistance of the materials. These tests and devices allow the analysis of material defects due to corrosion and the quality assurance of the selected material, while at the same time providing input for the development of new technological materials, paint and coating methods.
The most general form of corrosion is that the materials become inoperable in time by degrading the environment. The life of the material is directly related to corrosion resistance. It is important to know the time of corrosion to know about the service life of the materials. In order to determine the corrosion resistance of the materials, corrosion tests have been determined according to different coating types and thicknesses in the standards established by different disciplines all around the world and test devices have been designed to perform this corrosion tests. According to the coating type and coating thickness, the tests carried out under the conditions specified in the aforementioned spesifications are used to obtain information about the corrosion resistance of the material. The environmental impacts (moisture, heat, temperature, salinity) required for corrosion can be partially achieved under experimental conditions, but there are still limitations about time. Corrosion tests durations are still very long and for the development of high reliability tests and devices in a short time, studies on accelerated corrosion testing are still in progress.
Corrosion testing is one of the most important aspects of corrosion control because it is used to advanced technology and to determine the most effective economical meansto achieve acceptable performance.A large number of factors effect corrosion behavior.ASTM and other diciplines such as NACE, ISO and other have standardized many test and practices. These tests are designed for different test conditions to show coated materials corrosion performans. Each tests salt solution percent and content are different and performed at different temperature. The principle and widely used test that show corrosion resistance performance is salt spray fog tests that used commonly for Zinc coating materials.
Salt Spray corrosion testing (per ASTM B117) is done in a controlled test chamber at stated temperature on standard with continuous exposure to an atomized fog of salt solution. Salt spray test results provide  a method of comparison testing of coatings in a controlled environment, but may not directly predict corrosion protection in real world conditions. The standards were just consulted  for specific minimum salt spray testing requirements.
Salt spray test conduct in two phases. Related standarts meausure white corrosion and red rust observation time according to coating type and thickness .In direction of duration time, coating and material's service lifes can be predicted. White corrosion means the Zinc coating decay and it shows pasivation performance on the other hand Zinc coating corrosion resistance. 
When zinc corrodes, it develops a white powdery product on the surface, analogous to and just prior to the appearance of red rust on steel. Because when corrosive test chamber totally eliminate coating layer and reach base material, red rust were observed. That means Zinc coating dedicated itself for protecting base material and gave out totaly on base material surface. From this time red rust was observed and it means main metal will be wasted under corrosive enviroment.Red rust and white corrosion time are main indicators of salt spray testing.
In this study, the corrosion resistance of the coating on electrolytic galvanized coating plates, which is one of the zinc coating techniques, was investigated. By investigating the effects that would speed up corrosion, tests were carried out under different experimental conditions by changing the parameters that would accelerate the corrosion in line with the researches. The effects of temperature, salt content and pH parameters of the 7 micron thickness Zinc plated flat plates on the temperature, salt ratio and pH parameters of the corrosion protection tests were observed.
Each expiremental studies different from each others and references expiremental setup was designed in direction  ASTM B117 and FIAT Norma 5018 Salt Spray Fog Corrosion Test requirements. Electrolytic galvanized coating plates that used on this study were prepared  in direction of standars parametres based on ASTM B633 and FIAT Norma Harmonized PS.50036 as Fe/Zn IV designation. All tests were perfomed with same plates that coated in same process and undergone same pasivation exposure time so every plate have same coating thickness. Plates are also have same dimeonsions and made of same base material to eliminate any potantial deviation comes from other factors.Coating thickness of plates was verified with X-ray devices and used equipments were not changed till the end of tests completion. Briefly, all parametres were fixed and just test conditions are desing with different parametres for observed its effects to corrosion times. 
24 different test conditions were set in the beginning of exprimental study but due to the constraints for time and cost, experiments were carried out using the minimum, maximum and mid-points of each of the variable parameters. The experimental design was done by using the Minitab Package Program and the design results were analyzed by using the same program. DOE methods decrease the quantity of test conditions (11 different test conditions) with using statistical method and calculate other test condition corrosion times. Thanks to DOE(Desing of Expirement) method, the corrosion times under the test conditions, which were not made by the minimum and maximum values, were statistically obtained with certain reliability values and the formulas were removed by the program for corrosion times.
The 11 different expirements were carried out with reference to the nominal condition specified in the norms and the white rust and red rust formation time was recorded and this study was based on the principle of how the other test performed by changing temperature, pH value and salt ratio accelerated compared the reference test.
The most improtant parameter to effect white corrosion and red rust is the high temperature.The sequencing can then be done as a pH value and a combination of temperature and pH respectively.
The effect of salt content is much lower when compared to other parametres. At  high temperatures and low pH values, white and red rust formation increases, corrosion progresses much faster, but fluctuating values are observed for salt content. This fluctuation  was analyzed in the Minitab  - DOE Program and it was confirmed that the salt ratio P value ( possible margin of error ) in the system was high. 
As a result of the values made, it was found that the system has generality(reliability) of 72.85% for while rust formation and 88.51% for red rust formation.

The nominal test condition is taken as reference and highest temperature and salt ratio value with lowest pH value of the plates are corroded in the shortest time and with this study the corroison formation time is 91.44% for red rust and observed that it was shortened by 97,91% for white rust.