Pirinç alaşımlarının farklı sıcaklık ve tutma sürelerinde çinkosuzlaşma davranışlarının incelenmesi
Investigation of dezincification behaviours of brass alloys at different temperatures and holding times
Yazar: CANER KARADEMİR
Danışman: DOÇ. DR. NECİP ÜNLÜ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Pirinç alaşımları = Brass alloys

Özet:
Bakır-çinko alaşımı olan pirinçler, üstün korozyon ve mekanik dayanımları sayesinde özellikle içme suyu sistemlerinde yaygın kullanım alanı bulmaktadır. İçme suyu sistemlerinde pirinçlerin yaygın olarak kullanıldığı ana parçalar; valfler, bağlantı parçaları ve musluklar' dır. Daha iyi korozyon, mekanik ve işlenebilirlik özelliklerinin sağlanması amacı ile pirinç alaşım sistemleri geliştirilmektedir. Pirinç alaşımlarında yaygın olarak kullanılan alaşım elementlerinden biri, malzemenin işlenebilirliğine olumlu yönde katkı bulunduran kurşun' dur. Ancak kurşun içeren pirinç alaşımları, içme suyu sistemlerinde kullanıldığında insan sağlığını tehlikeye atmaktadır. Kurşun içermeyen pirinç alaşımlarının tasarımında kurşun miktarı ağırlıkça yüzde 0.2' den az olarak kullanılmaktadır. Günümüzde çinkosuzlaşma direnci olan ve kurşun barındırmayan pirinç alaşımları geniş bir kullanım alanı bulmakta olup, özellikle içme suyu sektöründe kurşun barındıran pirinç alaşımlarının yerini almaktadırlar. Pirinç malzemelerinde bulunan alaşım elementlerinin; değişken sıcaklık ve tutma sürelerinde, malzemenin çinkosuzlaşma direncine etkisini gösterebilecek sınırlı sayıda çalışma olması, bu çalışmanın özgün değerini yansıtmaktadır. Kurşun içermeyen CW511L, CW724R alaşımları ve kurşun içeren CW602N, CW603N, CW617N, CW625N alaşımları olmak üzere toplam 6 farklı alaşım üzerinde çalışılmıştır. ISO 6509-1 testinin uygulanması yanında farklı sıcaklık ve tutma sürelerinde davranışları da gözlemlenmiştir. Çinkosuzlaşmanın tespiti, korozyon oluşumu ve korozyon mekanizmasının ortaya çıkarılabilmesi için yapılan karakterizasyon çalışmalarında optik mikroskop ve taramalı elektron mikroskobisi (SEM/EDS) kullanılmıştır. Deneylerde her bir alaşım için 75°C-95°C sıcaklıkları ve 6, 12, 24 ve 48 saat zaman süreçleri kullanılmıştır. Numunelerde çinkosuzlaşma derinlikleri ölçülmüş, çinkosuzlaşan bölgeler incelenmiş olup alaşım elementlerinin çinkosuzlaşma dayanımına etkileri irdelenmiştir. İncelemelerde CW603N ve CW617N alaşımlarının çinkosuzlaşma direnci olmadığı ve artan tutma sürelerinde çinkosuzlaşma derinliklerinin arttığı görülmüştür. Diğer alaşımların ISO6509-1 standardına göre çinkosuzlaşma dirençli alaşımlar olduğu tespit edilmiş olup, artan sıcaklıklarda CW625N alaşımında çinkosuzlaşma görülmüştür. Artan kurşun ve kalay miktarlarında malzemelerde beta fazı oluşumu sebebiyle çinkosuzlaşma direncinin azaldığı gözlemlenmiştir. Ek olarak pirinç alaşımlarında alüminyum bulunması sebepli beta fazı oluşumu ve dolayısıyla çinkosuzlaşma görüldüğü de belirlenmiştir.

Abstract:
Brass has held significant importance across various civilizations throughout centuries. Especially in periods before the importation of gold and silver from the New World began, brass was utilized in the jewelry sector and as a construction material for church buildings. In Europe, particularly in Germany and Belgium during the 15th and 16th centuries, brass production saw significant advancements. In England, mass brass production started at Tintern Abbey in 1568, with the industrial distillation method being discovered by William Champion in 1738 and patented by James Emerson in 1781. However, due to economic reasons, this method only became widely used 70 years after its patenting.
Over time, the applications of brass expanded, and various brass alloys were developed after the mid-19th century. For instance, Muntz Metal was discovered in 1832, and in 1846, brass was used in France for military ammunition production (cartridge brass). In America, brass production began after the independence movement, particularly in the Waterbury and Connecticut regions, where clocks, buttons, and lamps were manufactured. The aesthetic qualities of brass facilitated its widespread use in wall clocks, wristwatches, chronometers, and compasses. Even today, brass continues to be a significant material in many industrial and commercial applications.
Copper-zinc alloys known as brasses are widely used in drinking water systems due to their excellent corrosion resistance and mechanical properties. They are commonly used in valves, fittings, and faucets. Brass alloys are developed by varying the amounts of copper, zinc, and other alloying elements, resulting in different properties such as corrosion resistance, mechanical strength, and machinability. Using brass alloys without adequate dezincification resistance in drinking water systems can lead to significant costs due to corrosion-related failures. These costs include water loss, decreased trust in the manufacturer, and mold propagation. A notable example is a case in Nevada, USA, where improper brass alloys led to leaks and failures in water systems affecting over 30,000 homes, resulting in a $100 million settlement. Over the years, the dezincification issue in drinking water systems was thought to be resolved by using brass alloys with lower zinc content. However, the demand for easily machinable and cost-effective brass alloys has led to the use of high-zinc content alloys, necessitating the addition of other alloying elements to maintain dezincification resistance. Lead has been considered a key alloying element to prevent dezincification, but its introduction into drinking water poses health risks. Lead is an important alloying element in brass materials known to facilitate machining in metalworking. However, its use in alloys intended for drinking water systems poses risks. To minimize potential effects, global regulations, endorsed by organizations such as WHO and the European Commission, stipulate that lead concentrations should not exceed 10 ppb (μg/L). Consequently, brass manufacturers are focusing on developing new lead-free alloys (< 0.2% wt Pb) that offer good corrosion resistance and machinability.
Dezincification remains a fundamental corrosion issue in brasses used in drinking water systems. Restrictions on lead alloying necessitate the development of new alloys, although challenges persist in ensuring their long-term durability in these systems. This has spurred further research needs into the dezincification behavior of brass materials. ISO 6509-1 is the primary standard for detecting dezincification and measuring dezincification resistance. However, discrepancies between corrosion data obtained from standard tests and laboratory-produced results have raised questions about underlying mechanisms in these new brass classes. Therefore, detailed investigations into the corrosion behavior of lead-free brass alloys are crucial. Such studies will enable these alloys to serve as viable alternatives to leaded brasses, minimizing significant losses in corrosion and mechanical performance.
Brass is a copper-zinc (Cu-Zn) alloy containing 5-40% zinc by weight, with zinc being the primary alloying element. When the zinc concentration is up to 35%, the material, which contains a minimum of 63% copper and has a fine microstructure represented by the α-phase, is characterized. This class of α-brasses, also known as cold-worked brasses, offers good ductility at room temperature, facilitating cold forming processes. Compared to brasses with higher zinc content and a zinc-rich second phase, α-brasses exhibit higher corrosion resistance. However, their workability diminishes at high temperatures, and their high copper content increases costs, thus limiting their applications. Brass alloys used for commercial purposes are classified into two main groups based on their structure: α and α + β brasses. This distinction depends on the zinc content of the alloy. For instance, brass alloys containing less than 39% zinc consist solely of the α phase, while those containing around 39% zinc exhibit both α and β phases. The β phase is harder and stronger than the α phase but also more brittle. Increasing zinc content enhances the strength and hardness of the brass alloy but reduces its ductility. Therefore, as the zinc content increases, the mechanical properties of the alloy change. Particularly in alloys containing 39-47% zinc, the presence of the β phase leads to increased tensile strength and hardness but decreased ductility. This influences the preference for different brass alloys depending on their intended applications. β brasses are ideal for applications requiring high strength, although they require hot working due to their lower ductility at low temperatures. β brass is the type with the highest hot working capability and is commonly preferred in industrial applications. A brass alloy containing between 47-51% zinc by weight achieves the highest strength value with a pure β phase. This type of alloy is particularly favored for applications demanding high strength. A brass with approximately 50% zinc can transform to 100% β phase when rapidly cooled from high temperatures. These properties enable the utilization of different brass alloys in various industrial and commercial applications. Brasses are known for their good corrosion resistance due to their high content of the valuable metal Cu. However, brass also contains a significant amount of the less valuable metal Zn. When a valuable metal, in this case, Cu, interacts with a less valuable metal like Zn, Zn will preferentially corrode more quickly and migrate into the solution. This corrosion mechanism in brass is known as dezincification. Various forms of corrosion such as dezincification (alloy separation), stress corrosion cracking, and intergranular corrosion are observed in brass. This thesis investigates the dezincification behavior of brass alloys used in drinking water systems.
The aim of this thesis is to assess the dezincification resistance and corrosion behavior of six different brass alloys under varying temperatures and holding times. The study identifies alloys that pass ISO 6509-1 standards and examines their behavior under different conditions. It also investigates alloys that fail under standard conditions to assess their performance under lighter and alternative conditions. Experimental durations of 6, 12, 24, and 48 hours at temperatures of 75°C and 95°C were utilized. Depth measurements and chemical composition analysis of dezincified regions were conducted according to ISO 6509-1 standards to determine dezincification behavior. The goal is to classify alloys based on their dezincification resistance and identify the most durable alloy. Factors influencing corrosion resistance are also investigated.
Experiments were conducted on six different brass alloys at two temperatures, 75°C and 95°C, and for four different durations: 6 hours, 12 hours, 24 hours, and 48 hours. In total, 48 samples underwent corrosion testing according to ISO 6509-1 standards. Subsequent characterization studies were performed using optical microscopy and SEM/EDS analyses. The brass rods with different chemical compositions were provided and labeled according to their commercial names. The brass alloys used in the experiment were coded as CW511L, CW602N, CW603N, CW617N, CW625N, and CW724R and were monitored throughout the experiments. The alloys were supplied as rods and hexagonal bars.
The brass materials were cut using a saw to achieve a surface area of 100 mm² according to the ISO 6509-1 standard. The height of each cut sample was 10 mm, resulting in a total of 48 samples. The cut samples were mounted in Bakelite. The samples were then ground sequentially with 60, 300, 600, 1000, and 2400 grit sandpapers and polished using a 1-micron silicon carbide solution.
A solution was prepared by adding 25.4 g of CuCl2 powder to 2000 cc of DI water. The powder was mixed in a plastic container until fully dissolved in the DI water. Each set of 6 samples was placed in 800 cc of this solution.
The samples were placed in groups of 6 in a container, and 800 cc of the prepared solution was added to each container. The containers were sealed with aluminum foil tape to prevent air from entering. The oven was heated to 95°C, and the first set of 6-hour samples was placed in the oven. After 6 hours, the samples were removed from the oven and allowed to cool. Once cooled, the containers were opened, and the samples were removed with tongs and cleaned with alcohol to remove any acid residues. The containers were then washed. The same procedure was repeated for the 12-hour, 24-hour, and 48-hour sets of samples.
After obtaining optical microscope images of alloy cross-sections, the depths of dezincification were measured using scales. To ensure uniform measurement results for all alloys, dezincification depths were measured at 25 points on the microstructure, and the average and standard deviation of these values were calculated. The average dezincification depths of the alloys were then plotted on a graph for analysis. Dezincification was observed in CW603N and CW617N alloys during microstructure examinations. SEM/EDS analyses were conducted on CW603N, CW617N, CW511L, and CW625N alloys that underwent a 24-hour corrosion test at 75°C. Due to the depth of the corrosion layer, the CW603N alloy was examined at 500x magnification, while CW617N, CW511L, and CW625N alloys were examined at 1000x magnification. For better understanding, the samples were also supported with EDS analysis.
It has been observed that CW603N and CW617N alloys lack resistance to dezincification, with their matrix undergoing dezincification and reaching the highest depths compared to other alloys. If CW603N and CW617N alloys are used in drinking water systems, they will experience wear due to complete corrosion of the matrix, potentially causing blockages and water leaks in the long term.
CW511L, CW602N, CW625N, and CW724R alloys did not undergo dezincification at any retention time, making them the most reliable alloys for use in drinking water systems. However, CW602N alloy contains lead, making it unsuitable for drinking water systems. Therefore, CW724R alloy, which does not contain lead, is considered the most suitable.
The experiments did not reach a definitive conclusion regarding an increase in dezincification depth with rising temperatures. However, it was observed that increased retention times led to greater depths of dezincification.
The alloys were examined for the effects of elements on dezincification resistance. It was determined that alloys with dezincification resistance typically contain approximately 0.06 wt% arsenic. The presence of 0.10 wt% iron in the alloys was found to be insufficient to form an intermetallic compound with arsenic and did not negate the effect of arsenic on dezincification resistance. In the absence of arsenic, lead was observed to precipitate in the structure, accelerating dezincification.