Pirincin gerilmeli korozyonu

Abstract

Makina parçaları değişik türlerde hasarlara uğramaktadır. Bunlardan biri de metal ve alaşımların, içinde bulundukları ortamın etkisinden dolayı ortaya çıkan korozyon hasarıdır. Mekanik etkinin (gerilmenin) ve korozyonun parçaya birlikte etkimesi sonucu ortaya çıkan hasar da gerilmeli korozyon çatlaması olarak adlandırılmaktadır. Makina parçalarının tasarım aşamasında gerilmeli korozyon çatlaması özelliklerinin bilinmesi ve buna göre gereken tedbirlerin alınması gerekir. Malzemelerin gerilmeli korozyon çatlamasına karşı duyarlılıklarının tesbi tinde standar t laşmış değişik deney metotları k ul lanı 1 mak tadı r. Bu çalışmada pirincin gerilmeli korozyon çatlaması oluşum mekanizmaları incelenmiş ve PİS8 ve Pİ63 pirinci erindeki tane boyutu değişikliklerinin süneklik kaybına nasıl etkidiği araştırılmıştır. Ayrıca Pİ58 pirincinde hasar zamanının tane boyutu ile değişimi araştırılmıştır.

Stress corrosion cracking refers to cracking caused by the simultaneous presence of tensile stress and a spesific corrosive medium. Many investigators have classified all cracking failures occuring in corrosive mediums as stress corrosion cracking, including failures due to hydrogen embrittlement. However, these two types of cracking failures respond differently to enviromental variables. Cathodic protection is an effective method for preventing stress corrosion cracking whereas it rapidly accelerates hydrogen-embrittlement effects. The site of initiation of a stress corrosion crack may be submicroscopic and determined by local differences in metal composition, thicness of protective film, concentration of corrodent, and stress concentrations. A pre-existing mechanical crack or other surface discontinuity, or a pit produced by chemical attack on the metal surface, may act as a stress raiser and thus serve as a site for initiation of stress corrosion cracking. Several teories have been advanced to explain in detail the machanism of stress corrosion cracking. Two major teories are the electrochemical and stress sorption teories. According to the electrochemical theory, galvanic cells are set up between metal grains and anodic paths are established by heteregeneous phases. When the alloy, stressed in tension, is exposed to a corrosive environment, the ensuing localized electrochemical dissolution of metal, combined with localized plastic deformation, opens up a crack. With sustained tensile stress, protective films that form at the tip of the crack rupture, causing fresh anodik material to be exposed to the corrosive medium, and the stress corrosion caracking is propagated. This theory has been extended to include metals that do not form intermetallic precipitates, but for which phase changes or segregation of alloying elements or impurities can occur during the process of plastic deformation of metal at the crack tip, the resulting composition gradient then setting up galvanic cells According to the stress sorption theory, stress corrosion cracking generally proceeds by weakening of the cohesive bonds between metal atoms through adsorption of VII damaging substances* in the environment. The surface energy is said to b£ reduced, increasing the probability that the metal wili:; form a crack under tensile stress. Adsorption any kind of complex ions that reduce surface energy should favor crack formation. Susceptibility of a given metal to stress corrosion cracking in a specific environment depends on its ovei - all and local chemical composition and on its metallurgical structure. High-purity metals generally are much less susceptible than alloys. In general, binary alloys that contain only very small\ amounts of elements other than the two major constituents are quite susceptible to failure by stress corrosion cracking. Crystal structure also has an effect on stress corrosion cracking. For instance ferritic stainless steels (body-centered cubic) are much more resistant to stress corrosion cracking when exposed to chlorides in aqueous solution than ar,e austenitic stanliess steels C face-centered cubic). Stress corrosion cracking occurs only in the precence of tensile stresses, which may be externally applied or residual. In the purely compressive stresses condition stress corrosion cracking will not occur. The role of tensile stressing is important in rupture of protective films during both initiation and propagation of cracks, these films may be tarnish films C such as those on brasses), thin oxide films, or other passive films. Breaking of a film a head of an advancing crack permits crack propagation to continue in the metal. The surfaces of some stress corrosion cracks resemble those of brittle mechanical fractures although they actually are the result of local corrosion in combination with tensile stress. The path of stress corrosion cracking in metals is governed by composition and structure of metal and by the environment In some metals, cracking propagates intergranularly, and in others, transgranularly. In certain metals, such as iron-chromium alloys and brasses, either type of cracking can occur, depending on the metal -environment combination. VIII The ammonium ion generally considered to be the specific corredent causing stress corrosion cracking in copper base alloys in service. Water or water vapor, of course must be present ; otherv/hise, the ion is not form. Oxygen and carbondioxide are also considered to accelerate cracking in an ammonical atmosphere in laboratory and in service failures. There are also other corrodents that produce stress corrosion cracking. Brasses in aqueous sol utions, pH has a strong influence on susceptibility to cracking and on whether cracks paths are intergranular or transgranular. Brasses containing less than 15% zinc are generally considered very resistant to stress corrosion cracking. Susceptibility to stress corrosion cracking is generally believed to increase with the zinc content up to about 40%. Cold rolling proces influence the resistant of brass to stress corrosion cracking. After a small amount of cold rolling, the distorted metal is near the surface, with the possibility that there are high residual stresses at the surface. As the degree of cold work is increased, the susceptibility to stress corrosion cracking decreas. Several mechanism has been suggested to explain stress corrosion cracking of brasses. To explain intergranular stress corrosion cracking of a- brasses, film rupture mechanism has been suggested. Film rupture model is based on the concept that plastic deformation exposes film free metal at the crack tip, permitting crack propagation to occur by localized anodik dissolution. To explain transgranular stress corrosion cracking of ot and a+ft brasses, adsorption model, local anodik dissolution and film having brittle properties have been suggested. Failures are intergranular in a brass in following conditions; a- Alloys with <18%Zn in non-tarnishing solutions. b- In annealed condition and in tarnishing solution. IX Failures are transgranular in a brass in following condition; a- Alloys with >18Zn in non-tarnishing solutions. b- Heavily cold worked Cu-30Zn in tarnishing solutions. In a+ft brass stress corrosion cracking propogate transgranular ft and interphases. In the design stages of any structures, it is important whether any construction materrials are susceptible to stress corrosion cracking in environment used. To determine the stress corrosion cracking susceptibility of materials in laboratory condition some tests methods have been improved. Before 196S, constant-load and constant strain tests of smooth and notched test specimens of various configurations were used to asses stress corrosion cracking. During the 1960*s, two accelerated test techniques based on different mechanical approach emerged. One technique tests and analyzes statically loaded mechanically precracked test specimens using linear elastic fracture mechanics concepts. The second technique consists of constant (slow) strain rate tests on smooth or precracked specimens. Laboratory testing with these techniques frequently has produced stress corrosion cracking. Whereas constant -load and constant-strain have not. Slow strain rate testing always ends in specimen fracture. The most significant variable in slow strain rate testing is the magnitude of strain rate. If strain rate is too high, ductile fracture will occur before the necessary corrosion reactions can occur. However at too low slow strain rate corrosion may be prevented due to film repair so that the necessary reactions of bare metal cannot be sustained, and stress corrosion cracking may not occur. Slow strain rate testing does not provide data that can be used for design purposes. Principal methods of assesment of stress corrosion craking are based on parameters derived from slow strain rate tension testing, including time to failure, maximum gross section stress developed during the tension test, percet elongation, and reduction in area In this study, effect of the grain size on stress corrosion cracking behavior of a and a+ft brasses has been investigated, using constant strain rate and constant -load test methods. To show grain size effect on ductility loss slow strain rate test technique was used. It has been shown that grain size has no effect on the ductility loss of both a and a+ft brasses in Mattson solution C pH 7. S). According to the test results obtained from constant load test, time to fracture of a+ft brasses C Cu-40Zn) having small grain size is more than that of brasses having big grain size. Metal 1 ographic results show that cracks propogate transgranularly in ft phase, interphases between a. and ft, in a+ft brass. In a brass transgranular cracks have been determined.