Stress Corrosion with its Effects - Term Paper Example

STRESS COROSSION. Corrosion is the mechanical bit by bit eroding of a surface of a solid substance by either chemical reaction or friction. There are very many different mechanism of corrosion and some of the most notable include; oxidation, stress, galvanic, pitting and crevice just to mention a few. However in our study we are going to focus entirely on stress corrosion with its effects, how it is playing an important role in designing engineering components together with the current research being conducted towards elimination of this particular mechanism of corrosion. (Bill Nimo)Nobody if not few people in the world today expects properly designed products to fail or malfunction immediately it has been unpacked by the end user, but equally it is very unrealistic to expect the same product to last a life time, most end user expectations are that the product will work for a reasonable period of time that is enough to offset the cost of acquisition should it fail. When the lifetime issue becomes as a result of a product mechanical performance, design engineers arm themselves with appropriate tools to counter the deterioration of the mechanical product. A most likely factor is that most products are being designed to optimize performance, reduce the development cost and eventually take advantages of technological developments, but to the contrary, corrosion with its control is rarely accorded the proper attention it deserves yet it represent a very common failure. Even though SCC is very rare, its failure can be very costly and destructive when they occur. According to (Corrosion Doctors) Stress corrosion normally occurs abruptly after a products reliable period of satisfactorily service, this eventually leads to catastrophic failure of structures. SCC failures have very profound effects both to the user and the organization. It can reduce metal thickness which can lead to the loss of its mechanical strength causing failure or a breakdown. It might cause hazards to people arising from the structural failures. There is usually loss of time in availability of high profile equipments. There I the reduction of goods value due to deterioration of their appearances. There is contamination of fluids in vessels and pipes. Added complexity and expense of equipments due to designing it in order to withstand a certain amount of corrosion. There is constant mechanical damage to pipes, valves, or even blocking of the pipes by usage of solid corrosion substances. Typically SCC can be mainly spotted in pipe work, high pressure vessels, and highly stressed components and within systems that there is an abnormal operating conditions or the environment. The environment could either be permanent service or temporary e.g. cleaning of the product which could leave behind some residues. SCC is different from the normal corrosion from the sense that it requires pairing of certain material substances with certain environments for it to occur. However we need to note that SCC is usually not an inevitable process and for certain metals in particular environments it is very unlikely to occur, we therefore need to identify particular environment with specific metals that are required in order for SCC to occur. Table 1.0 Common SCC systems Material Environment Concentration Temperature Carbon Steel Hydroxides Nitrates Carbonate/ bicarbonate High Moderate Low High Moderate Low Low alloy steel water - moderate Stainless Steel Thiosulphate or polythionate low low Steel High strength Aluminium alloys Titanium Alloys Water vapor Chlorides Chlorides Methanol - Low High - Low Low Low Low This Table based on original classification from R. C Newman. Stress corrosion is usually a conjoint of three components; susceptible materials, sufficient tensile stress and specific chemical species. For stress corrosion to occur these three conditions must be met simultaneously. With temperature also playing a significant role in the environment that is necessitating crack, there is still no particular unified mechanism and some of the proposed various models are not limited to the absorption, film rapture model, pre-existing active path model and the embrittlement. It is also an insidious type of failure because mostly it occurs without any applied external load. (N.P.L)However the three basic mechanism of SCC are: Active Path Dissolution: - It involves accelerated corrosion alongside the path of higher than normal susceptibility, with most of the materials being bulky. One of the most popular active paths is the grain boundary; e.g. if an austenitic stainless steel is sensitized by precipitation of chromium carbide alongside grain boundary, the concentration of the local chromium at the grain boundary will be reduced. Applied stress will mainly occur to open up the cracks, allowing diffusion of corrosion products from the cracking tip which eventually enables the cracking tip to corrode faster. Hydrogen Embrittlement: - Hydrogen is a very small atom and it virtually dissolves in all the metals, this makes it easily fit in metal atoms within the crystals of most metals. Therefore it can diffuse more rapidly than the faster atoms. Hydrogen is normally attracted to regions with high triaxial tensile stress where there is dilation of the metal structure. It is systematically drawn within the regions ahead of notches that are under stress. This dissolved hydrogen will therefore assist in fracturing the metal, by developing an intense deformation of the local plastic. Film induced Cleavage. :- If let’s say a ductile material is coated with brittle film, then a crack being initiated within the film, might propagate within the ductile material for a small distance before eventually being arrested by the ductile blunting. If the brittle film was formed due to corrosive processes then it can be reformed on the cracked tip and the same process could be repeated. The brittle films that causes film induced cleavages are the de-alloyed layers such as brass, and the process of film induced cleavage usually results in trans-granular fractures. (N.P.L) Some of the widely known problems in SCC may include: Brass in Ammonia-containing environments. It was initially identified when the brass cartridge cases which were used by the Army of British in India were discovered to be cracking (The ammonia is as a result of the decay in organic materials). The main explanations given for the above process were that the cracks occurred during rainy season and secondly they resembled the cracks in seasoned wood. However that cracking is intragranular. Chloride cracking of Stainless Steel Stainless steel suffers from SCC in hot solutions that are known to contain chloride. There needs to be a very high concentrated solution of chlorides, even though small amounts of chloride in heated surfaces is sufficient for SCC to occur, the temperature at the time also needs to be around 70 degrees Celsius although it might occur below that in some situations, notably the acidic solutions. The cracking is bound to continue at very low stresses and this is usually attributed to residual stresses from fabrications. It is usually an transgranular and may sometimes switch to intragranular due to sensitization of the steel. Stress Corrosion Cracking of Steel Water Pipes. Most of cities water pipelines are made of asbestos, ductile iron, PVC, concrete and cast iron, however still there are quite a number of failures of water pipelines being reported across the world, the main reasons for the failure could be very diverse and complex and the diversification and complexity might be related to age, construction, pipe materials and the layout ground conditions. Water pipeline failure occurs due to circumferential cracking, longitudinal cracking, blow out holes, bell splitting, spiral cracking and bell shearing. Corrosion has been found to be playing a very important role in the blow out holes. When walls of the pipe have been reduced and thinned by corrosion to the point, the water pressure will be powerful enough to blow out the remaining thin wall. Circumferential cracking is also a common failure mode for the small diameter pipes, this is mainly due to some of the bending forces applied to the pipe and the small inertia associated by small pipes diameter (Maker 2001). When Baracos, (1995) analyzed pipe breakage data in Winnipeg, he clearly noted that expansive soil movement contributed to as much as 20 mm and would be responsible for around 0.5 % of the rotation at tightened joints. He also observed that the movements in pipes weakened by corrosion were responsible for join failures. In a separate study on “effects of cracking on water pipes” a commercial software was used in modeling the circumferential crack of a small diameter water pipe, a four point bending test was employed in studying the effects of cracking on water pipes, a pipe with a diameter 2 inches was used due to symmetry, it was later on concluded that the pipes bending capacity had been significantly reduced through wall cracks than by the partial cracks. Stress corrosion on Aircrafts components. SCC on aircraft parts may be due to a residual within the part itself, this usually occurs as a result of the production processes employed during manufacturing or even an externally applied cycle loading. Severe metal forming and tapered bolts are good examples of high residual tensile stresses and can eventually lead to SCC. The cracking of aluminium alloys that constitutes part of the pressure bulkhead of a certain vanguard aircraft, damaged and made the rear pressure bulkhead to rapture, this eventually resulted to losing a major portion of the horizontal tail surfaces during cruising flights. In this scenario corrosion initiated in places that could not be easily accessed to be inspected, this is a situation that frequently occurs in very many structures. Corrosion prevention. In order for corrosion to occur there must be certain factors, there is the susceptible material, environment to cause cracking of the material and finally a stress intensity factor should be present. From a technical view: 1. We should use corrosion resistant materials in manufacturing our products, either as a protective coating to cover reactive materials within the materials concerned. We should ensure the material is properly processed and fabricated to the standard. 2. We should also control the environment by manipulating its chemical composition. For instance we can remove or alter the environmental component responsible for the problem. 3. By changing the electrochemical factors that control corrosive reactions. Works Cited National Physical laboratory. Corrosion Control in Engineering Design. Corrosion Guides. http://corrosion-doctors.org/InternetResources/NPL.htm#Stress%20-%20complete Bill Nimo. National Corrosion Service. Corrosion Doctors. National Physical Laboratory. http://corrosion-doctors.org/InternetResources/NPL.htm#Stress%20-%20complete National physical laboratory. Basics of corrosion control. Corrosion doctors http://corrosion-doctors.org/InternetResources/NPL.htm#Stress%20-%20complete National Corrosion Control. Guide to good Practice in Corrosion Control. www.npl.co.uk/upload/pdf/stress.pdf Baracos. A Hurst, W.D. And Legget R.F (1955) “The effects of physical environment on cast ironed pipes” Vol 42. Pg. 1195-1206. Maker JM (2001) “Failure modes and Mechanism in Grey Cast irons” underground infrastructure research 2001, Canada, June 10-13. Read More