Stress Corrosion Cracking of Brass in Ammonia

Stress Corrosion Cracking of memorial tablet in ammoniaSix tastes of hardihood were taken, stamped to induce peacefulness period filter, were then exposed ammonia and ammoniacal copper sulphate solutions of different concentrations and composition for a period of 4 days to study the eroding characteristics of brass in ammonia. Five leavens were immersed in the solution and one of the samples was susp devastationed higher up solution. On visual examination of the samples revealed some corrosion products with striving corrosion ginger snap on the coat of brass. Microscopy analysis showed that the non- hang sample which was exposed to ammonia vapour in presence of moisture and oxygen produced high susceptibility to SCC than immersion samples in aqueous ammonia. The cupric ion present in the solution appears as an oxidant that provides a cathodic reaction and induces accent corrosion centering of brass in ammonia and ammoniacal solutions.INTRODUCTIONThere are three fact ors under which stress corrosion cracking is possible a) corrosive medium/ milieu b) poppycock and c) tensile stress. The corrosive medium to induce stress corrosion cracking depends on the potential, pH and temperature. The stress can be applied stress or residual stress. Stress corrosion cracking can occur in lower stress also hence it is the environment which plays a major role. legion(predicate) studies carried out revealed that failures of brass occur mainly in moist builds where ammonia, water and oxygen are present. Time of cracking depends on the pH of the solution. Dezincification of copper intermixture is possible when the zinc content present is sufficient enough.The aim of this experimental work was to investigate the corrosion behaviour of brass in ammonia.EXPERIMENTAL cognitive processSix brass samples and five test solutions were used in the experiment to study the stress-corrosion cracking of brass. The brass samples were labelled A, B, C, D, E, and F following t he residual stress given to them by stamping these letters into their surfaces using drill bits. The stamp was done at one end of the samples while the unstressed part served as the overcome for the experiment. The solutions serving as the test environments were also labelled using the letters 4A, 4B, 4C, 4D, 4E, 4F and their compositions are shown below.Compositions and colours of test solution sight before immersion sourceLabelCompositionColour observed beforesample immersionAAmmonium Sulphate with copper (II) 0.5M (NH4)SO4 + 0.05M CuSO4Pale blueBAmmonia solution with copper (II) 2.5M NH4OH + 0.05M CuSO4Dark blueCMattsons solution at pH of 7.2Medium blueDModified Mattsons solution containing 5% by volume of 0.1M NaClMedium blueE1M Ammonia solutionColourlessThe labelled samples were placed in the glass jar provided and five of the test samples were make full with enough test solution (10mm) to ensure complete immersion of the sample. Excessive alteration of the concentration of the NH3 solution was avoided by covering the top of the containers. The sixth sample was suspended in a higher place solution E (1M NH3 solution) in a glass jar (4F) by means of a surgical nylon string.After sufficient movie of the samples to the solutions for a period of four days, the samples were removed. The samples were visually examined first upon removal from the test solutions. The colours of the test solutions and corrosion products were recorded before the test samples were cleaned. The test samples were cleaned and rinsed in the pickling solution of 1M sulphuric acid solution in order to remove corrosion products formed on the surface of brass followed by rinsing with deionised water. in conclusion the samples were dried with tissues before being examined under an optical microscope for stress-corrosion cracking.RESULT AND DISCUSSIONsampling observation after scene of 4 days to test solutions runLabelTestObservation from visual examination and optical microscopyReason 1Brass sample immersed in solution ASolution remained lookout blue. No cracks were seen.corrrosion products found. Localized Corrosion damage at the stressed area observed back of the sampleStress-corrosion cracking may occur with further exposure to the solution.2Brass sample immersed in solution B for 4 daysSolution was tenebrificer compared to the original solution. Brass sample in tarnish condition (dark brown colouration). Crack at the stressed end.Stress-corrosion cracking of the sample.3Brass sample immersed in solution CNo significant change in solution colour. No cracks were seen.corrrosion products found.Sample shows low possibilities to SCC4Brass sample immersed in solution DDark brown corrosion products. Localized corrosion damage at the stressed end. No cracks seen.Stress-corrosion cracking may occur with further exposure to the solution.5Brass sample immersed in solution ESolution changed from colourless to light pale blue. Cracks seen.Colour change is likely due to formation of cupric ion, Cu2+. Stress-corrosion cracking at the stressed end.6Brass sample suspended in a higher place solution ESolution remained colourless. Tarnish film covering almost the entire surface. Cracks seen at stressed regionStress-corrosion cracking of the sample at the stressed end. Tarnish film likely to be cuprous oxide, Cu2O.Sample A-A-1 A-2Fig 1 Sample A immersed for 4 days in solution Ammonium Sulphate with copper (II)0.5M (NH4)SO4 + 0.05M CuSO4 poster No stress corrosion cracking observed at the stressed end of the sample Fig A-1.some corrosion product was noticed on the back side of the sample Fig A-2.Sample B-B-1 B-2Fig 2 Sample B immersed for 4 days in solution Ammonia solution with copper (II)2.5M NH4OH + 0.05M CuSO4OBSERVATION Stress corrosion cracking observed at the stress end and near the unstressed region just near to the stressed end Fig B-1.Dark corrosion product observed on the backside of the sample Fig B-2.Nature of cracks IntergranularSample C-C- 1 C-2Fig 3 Sample C immersed for 4 days in Mattsons solution at pH of 7.2OBSERVATION No cracking or both other significant changes observed in this sample after even 4 days of exposure. Only dark corrosion products were observed on the sample.Sample D-D-1Fig 4 Sample D immersed for 4 days in Modified Mattsons solution containing 5% byvolume of 0.1M NaClOBSERVATION No cracking or any other significant changes observed in this sample after even 4 days of exposure. Only dark corrosion products were observed on the sample.Sample E-Fig E-1 Fig E-2Fig 5 Sample D immersed for 4 days in 1M ammonia solution.OBSERVATION Stress corrosion cracking observed at the stress end extending to the edges and Dark corrosion product observed on the backside of the sample Fig E-1 and E-2.Nature of cracks IntergranularSample F-Fig F-1 Fig F-2Fig 5 Sample D suspended above in 1M ammonia solution for 4 days.OBSERVATION Stress corrosion cracking observed at the stress end extending to the edges and Dark corr osion product observed on the backside of the sample Fig F-1 and F-2.Nature of cracks Intergranular.From Optical microscopy analysis it is evident that sample B (Fig 2 B1B2) immersed in a test solution containing Ammonia solution with copper (II)2.5M NH4OH + 0.05M CuSO4 , Sample E (Fig5-E1E2) suspended above the test solution of 1M Ammonia solution and Sample F (Fig6 F1F2) suspended above the test solution of 1M Ammonia solution had under gone stress corrosion cracking and shows that only in some particular environment SCC occurs. Apart from stress corrosion cracking dark tarnish corrosion product was observed which could possibly be oxide layer of copper.Rest of the three samples A (Fig1 A1 A2) immersed in a test solution containing Ammonium Sulphate with copper (II) 0.5M (NH4)SO4 + 0.05M CuSO4 ,C (Fig 3 C1C2) immersed in a test solution of Mattssons solution at pH 7.2 and D (Fig 4 D) immersed in a test solution of Modified Mattsons solution containing 5% byVolume of 0.1M NaC l did not show any cracking but had tarnish corrosion product. These samples suffered localized corrosion at the stressed edges which can be seen in back side of the above 3 samples (Fig A2 C2).The electrochemical reactions of brass in ammonia environment are as followsCopper free aqueous ammonia is oxygen reduction and primary oxidization reaction is oxidation of copper to form cuprous intricate ions.Cu + 2NH3 Cu (NH3)2+ + e-The cuprous complex ions formed further reacts with oxygen forming cupric complex ions2Cu (NH3)2+ + 1/2 O2 + H2O + 4NH3 2Cu (NH3)42+ + 2OHCuprous complex ion cupric complex ionThe formation of cupric complex ions leads to stress corrosion cracking of brass in ammonia solutions. The process does not proceeds indefinitely since the attack of cupric complex ion (oxide layer) occurs at critical copper ion content which leads to decrease in corrosion rate but the cracking continues with the copper ion concentration.It is also evident from the black dark coloured product formed (Tarnish colour) on the surface that the cracks are intergranular in nature.The pores on the surface of the corrosion product were the sites for localized attack, likely to survive at the grain boundaries. In general the stress corrosion cracking in brass has considered being intergranular. On absence of the black coloured corrosion product it is believed that the cracks to be transgranular nature of crack.CONCLUSIONIt is evident from the discussion carried out above that brass is susceptible to stress corrosion cracking in ammonia. Stress corrosion cracking was observed in the regions where residual stresses were generated leaving behind the rest of the areas which were not induced to stress. Formation of cupric complex ions leads to stress corrosion cracking of brass in ammonia solutions.The cracks formed on the brass surface can be further studied under SEM to provide detailed information on the nature of the crack formed.In order to overcome the above problem of stress corrosion cracking the following options can be optedEliminating any one of the primary factor (Environment, material and stress) will mitigate stress corrosion cracking.Use of more cracking resistance alloys such as Cu-10Ni instead of Cu-Zn