Pirinç alaşımlarının çinkosuzlaşma korozyonuna alüminyum, nikel ve kalay alaşım elementlerinin etkileri
The effect of aluminium, nickel and tin additional elements on dezincification corrosion of brass alloys
Yazar: ATALAY YAZAN
Danışman: PROF. DR. MAHMUT ERCAN AÇMA
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bakır çinko alaşımı olan pirinç M.Ö 500'lü yıllarda Çin'de üretilmiş ve kullanılmıştır. M.Ö 300 ve 200'lü yıllarda ise Orta Asya'da pirinç alaşımlarının kullanıldığı bilinmektedir. Günümüzde halen birçok alanda kullanılmakta olan pirinç alaşımları üstün işlenme özelliği, iyi ısı ve elektrik iletkenliği, korozyon dayanımı ve çekici rengiyle günlük hayatımızda da birçok yerde karşımıza çıkmaktadır. Pirinç alaşımlarının korozyona karşı dayanımı iyi olmasına rağmen, sulu ortamlarda görülen bir çeşit seçici korozyon türü olan ve çinkonun selektif olarak çözünerek alaşım yapısından uzaklaşması ile meydana gelen çinkozsuzlaşma korozyonu pirinç alaşımları için sorun teşkil etmektedir.
Bugüne kadar çinkosuzlaşma korozyonunu önlemek adına çeşitli yöntemler kullanılmıştır. Bu yöntemler; farklı alaşım elementi ilavesi, ısıl işlem uygulamak, inhibitor ilavesi ve yüzey işlemi uygulamak gibi tekniklerden ileri gelmektedir. Fakat bu yöntemler içinde kitlesel üretime uygunluğu ve maliyetinin diğer yöntemlere nazaran daha düşük olması sebebiyle alaşım elementi ilavesi en yaygın ve en etkili yöntem olarak karşımıza çıkmaktadır.
Alaşım elementi ilave edilerek çinkosuzlaşmayı durdurma üzerine yapılan çalışmalar alaşım içindeki çinko miktarına bağlı olarak büyük değişiklikler göstermiştir. Alfa yapıdaki pirinçlerin, dubleks yapıdaki ve beta yapısındaki pirinçlere göre daha az çinko içermeleri bu türü çinkosuzlaşma korozyonuna daha dayanıklı hale getirmektedir. Fakat bu alaşımların işlenebilirlikleri sınırlıdır ve sıcak işleme uygun yapıda değillerdir. İşlenebilirliği kurşun ilavesi ile arttırılan ve sıcak şekillendirme prosesine uygun olan dubleks yapıdaki pirinç alaşımları bataryalar, sıhhi tesisat sistemleri, kondanser borular gibi sulu ortamlarda çalışan sistemlerde kullanılmaktadırlar.
Bu çalışmada dubleks yapıdaki pirinç alaşımlarında çinkosuzlaşma davranışına alüminyum, kalay ve nikel alaşım elementlerinin etkileri incelenmiştir. Pirinç alaşımlarına farklı yüzdelerde ilave edilerek bileşimin etkisi araştırılmıştır. Deneyler 3,5%NaCl ortamında yapılmış -0,4, +0,2 V potansiyel aralıklarında tafel eğrileri elde edilerek korozyon davranışları incelenmiştir. Deney sonunda solüsyon atomik absorbsiyon spektrometresinde analiz edilerek çözeltiye geçen bakır ve çinko miktarları saptanmış ve çinkosuzlaşma faktörü hesaplanmıştır. En iyi korozyon direnci gösteren alaşım türleri SEM ve EDS analizleri ile kıyaslanmışlardır.
Çalışmanın sonucuna göre Sn ilavesi %3'e kadar çinkosuzlaşmayı önlemede etkili olmuştur. Bu değerden sonraki ilaveler çinkosuzlaşma hızını arttırmıştır. Alüminyum ilavesinin çinkosuzlaşma üzerinde belirgin bir etkisinin olmadığı anlaşılmıştır. Nikel ilavesi ise çinkosuzlaşmayı önlemede başarılı olmuştur. Artan nikel ilavesi ile çinkosuzlaşma hızının ters orantılı olarak değiştiği saptanmıştır.

Abstract:
Brass is a copper zinc alloy and it was found and  produced during BC 500 in China. In the BC 300 and BC 200 brass is known to used in Central Asia. Brass has been made for almost as many centuries as copper but has only in the last millennium been appreciated as an engineering alloy. 
Initially, bronze was easier to make using native copper and tin and was ideal for the manufacture of utensils. Predynastic Egyptians knew copper very well and in hieroglyphs copper was represented by the ankh symbol, also used to denote eternal life, an early appreciation of the lifetime cost-effectiveness of copper and its alloys. While tin was readily available for the manufacture of bronze, brass was little used except where its golden colour was required. The Greeks knew brass as 'oreichalcos', a brilliant and white copper.
Because of outstanding processing capability, good thermal and electrical conductivity,corrosion resistance and attaractive color. Nowadays, brass material are still used in many areas such as batteries, condenser pipes, coins, music instruments, bullets, propellers.
Brasses are widely used as constructional material in marine environment due to their anticorrosive, antifouling and mechanical properties. Despite their resistance of corrosion, dezincification behaviour is problem for brasses and it occurs in brass materials under aqueus systems and it exists with dissolution of zinc. 
Brass components are  also widely used in drinking water distribution systems as valves, faucets and other fixtures. They can be corroded by "dezincification," which is the selective leaching of zinc from the alloy. Dezincification in potable water systems has important practical consequences that include clogged water lines, premature system failure and leaks, and release of contaminants such as lead.
An in-depth literature review revealed that conventional wisdom about dezincification was no longer adequate in explaining failures observed in modern water systems. Little research has been conducted since the landmark work of Turner et al. nearly half a century ago. The potential role of chloramines, phosphate inhibitors, and modern understanding of water chemistry need evaluation. The role of physical factors including stirring, heating and galvanic connections are also potentially influential. 
A mechanistic study of zinc solubility and corrosion of copper: zinc couples provided insight to factors that might mitigate and exacerbate zinc leaching from brass. Zinc solubility and corrosion was reduced by higher pH and bicarbonate, but was enhanced by higher chloride. Hardness ions including Mg+2 and Ca+2 had little effect.
Alloys with higher zinc content had a greater propensity for dezincification corrosion. Stirring and galvanic connections caused brass to leach more metals and have higher weight loss. Heating may contribute to corrosion scale accumulation. 
A comprehensive examination of dezincification as a function of water chemistry used numerous techniques that include measurement of galvanic currents, metal leaching, and weight loss. In general, as would be predicted based on results of the study of solubility and corrosion of pure zinc, chloride emerged as an aggressive ion whereas bicarbonate was beneficial to brass corrosion. Hardness had little impact, and phosphates, silicates and Zn+2 inhibitors had a significant short-term benefit but little long-term benefit.
Some various methods have been used for preventing dezincification corrosion. Some of these methods are ; addition of diffrent alloying elements, addition of inhibitor to corrosive medium, heat treatment and surface treatment to protect dezincification.
Heat treatment has not been used commonly  until 1950's because of financial reasons. Method of heat treatment is expensive way for protection of brasses. On the other hand adding inhibitor to corrosive medium  provides limited using area.
In these methods addition of diffrent alloying elements is the most convenient one for churnig out  and it also the most  economic one for protection.
The investigations on dezincification corrosion by adding alloying elements have shown major changes depends on  amount of zinc content. Because of their less zinc content the type of alpha brasses are more resistant than dublex type and beta type brasses. However, the malleability of these elements are limited and may not be suitable for hot processing. In order to get machinability lead adds to dublex type brasses and it provides perfect machinability and it makes alloy suitable for hot processing.
Brass materials which used in aqueus medium produced with extrusion methods. Thus, dublex type brasses are prefered as a material for these conditions but this kind of material shows low dezincification resistance. They need extra process or extra additives to become resistant to dezincification corrosion. Previous studies 0.04%As was used as a additional element for preventing corrosion. Arsenic provides perfect dezincification resistance. However, arsenic is a toxic element and it is dangerous for health.
Hereby, our aim was to investigate alternative additional elements for preventing dezincification and also understand the effect of aluminium, tin and nickel on dezincification behaviours of dublex type of brasses.
In this study, the effect of aluminum, tin and nickel have been investigated as a additional elements under 3,5%NaCl medium. Experiments were performed under pH:5,5 medium and room temprature. Different percentage of these alloys were used to determine corrosion rate and dezincification behaviour. Each type of brasses were prepared between with %1-%5 additional alloys. For electrochemical studies, samples of 1cm2 area were sectioned and mounted in a cold setting epoxy after soldering a conductive wire to the back side of the samples. The specimens were polished mechanically using emery paper downt to 4/0 grade, degreased with acetone, washed with distilled water and dried.
In order to examine corrosion characteristic a three electrode  electrochemical cell was used with saturated calomel electrode (SCE) as a reference electrode and platin wire as counter electrode. Tafel curves were carried out using potentiostat interfaced to a personal computer using and analyzed using Gamry Echem Analyst Programme. Electrochemical measurement were performed by starting the potential scan -0,4V to +0,2 V with scan rate 1mV/s.  End of the test tafel curves were extrapolated and Ecorr, Icorr value calculated using Origin pro 8 programme.
During the characterization studies atomic absorption spectrofotometer (AAS), scanning electron microscope (SEM) and X-ray energies (EDS) were used to determine the nature of corrosion products. 
After corrosion tests, 100 ml sample from corrosive medium were taken to calculate dezincification factor (Z). Small aliquots of solution were periodically taken and analyzed using an atomic absorption technique. SEM and EDS analysis were used to compare three different samples which shows the best resistance to dezincification.
SEM and EDS analyses were performed to three samples  with corrosion products and after removal of corrosion product. These tree samples are; MS 58 %3,65Sn, MS 58 %2,42 Al and MS 58 % 5,23 Ni . The corrosion products were removed by immersing the specimens in sulfuric acid and potassium dichromate solution (ISO/DIS 28407.3). The samples were washed several times with distilled water, degreased with acetone and dried.
SEM expreriments shows us unsolved lead phase and some small particles on the corrosive surface depents on lead. After removal of corrosion products SEM pictures nearly as good as original samples.
We understand the analysis of corrosion products after EDS experiments. Before removal of corrosion products Cl and O elements give peak depending on corrosive medium which include NaCl. After removal of corrosion products these peaks were dissappeared. So, intensity of these peak directly related with corrosion rate.
According to experimental studies, addition tin to brass until % 3,65 percentage to alloys showed a good sustainability to dezincification. Addition of Sn increased the resistance to dezincification by the formation of Sn containing passive film on the surface. However, increasing tin content from percentage of % 3,65  increases the corrosion rate depends on a intermetallic phases. Optimum condition obtained for 60Cu32Zn3,65Sn2Pb alloy. The effect of aluminum shows different results and the addition of aluminum to dublex brasses did not significantly effect the dezincification rate. Increasing amount of aluminum in alloy shows a smilar results. It provides general corrosion protection but not a prevent dezincification. Best results taken with MS % 2,42 Al sample. However, in general we failed to prevent dezincification with using Al element as good as tin and nickel.
The best results are taken from nickel samples. With increasing nickel content dezincification rate was decreased linearly. Nickel in alloys behaves like a copper and nickel containing passsive film also inhibits dezincificication rate. Brass alloys with nickel behaves like a alpha brasses which is resistance to dezincification.