Stress assisted corrosion

This booklet talks about different types of stress assisted corrosion including stress corrosion cracking, hydrogen embrittlement, Hydrogen and stress assisted degradations, corrosion fatigue and others

Industrial corrosion failure: Case studies

The significance of corrosive processes on industrial failures is possibly less appreciated in our country than other industrialized nations. Whereas, there has been a proliferation of corrosion consultants in developed countries, it is only of late that we see the Indian industries realising the need for specialized treatment of their corrosion problems. Indeed, corrosion failures are insidious, and universal in occurrence. The forms and manifestations of the corrosive processes are too numerous. Moreover since corrosion is a function of alloy composition, environment, com¬ponent design, metallurgy, temperature and a host of other factors, generalization of corrosive processes and subsequent modelling proves to be futile. Thus corrosive failure analysis often turns out to be an exercise where component and industry specific diagnostic tools have to be employed, so that the failure analysis is often unique for the particular component tested. The solution of corrosive failures thus requires an understanding of the process environment, and sometimes an examination of the related processes, in addition to whatever information that can be gleaned from the affected component itself. The latter often requires thorough microscopic examination of the damaged part. Quite significant information about the corrosive process can often be found in the footprints left by the process, that is, the corrosion product or debris left near the site of corrosive attack. The analysis of the fluid environment that was in contact with the affected component is also often necessary. The microstructural examinations reveal irregularities in the component metallurgy as well as throw light on the physical manifestations of the corrosive pro¬cess. Environmental analysis yields clues on the corrosive species responsible for the process. Corrosion product analysis helps in determining the chemistry as well as mechanism of the process. So, these exercises carried out simultaneously often reveal enough information about the damage mechanism. The present paper illustrateds how the diagnostic methods can be employed to failure analysis through two case studies

Notch effects in corrosive failure

Notches act as stress raisers in any application. There are a few other detrimental effects of notches when an environment is involved. For a simple corrosive environment, the notch chemistry and electrochemistry are significantly different from that of the bulk environs. When hydrogen embrittlement is a possible failure mode, the presence of notch has an added significance, in defining the hydrogen transport and distribution. This article discusses the notch effects in general as it pertains to corrosive failure and hydrogen embrittlement in specific. The comparative studies on different notch geometries reveal the importance of notch dimensions in the defining the extent and morphology of cracking in the presence of hydrogen. A rationalization has been attempted involving notch hydrogen availability

Fundamentals of corrosion and its prevention

Most metals other than the noble metals, do not exist in their native states but exist as compounds of the metals (oxides, sulphides, etc.) indicating that such states are energetically more favourable. It is no wonder therefore that most metals tend to revert back to their more stable compound state through corrosion. Corrosion is thus inevitable for metals and alloys. Corrosion is the most predominant cause of metal failures today, surpassing other failure modes like fatigue, creep, impact, and others. The cost of corrosion is not only the cost of replacement but additional costs as well such as: ➢ Loss of production due to shutdown or failure > High maintenance costs ➢ Compliance with environmental and consumer regulations ➢ Loss of product quality due to contamination from corrosion of the materials ➢ High fuel and energy costs as a result of leakage from corroded pipes > Extra working capital and larger stocks Corrosion is differentiated from chemical attack in that it normally involves two or more electrochemical reactions. Thus while an insulating material can react with a strong chemical, such a degradation would not be considered corrosion as there are no electrochemical reaction involved. Any degradation suffered by a chemical or component resulting from electrochemical reactions involving species of the constituent material is corrosion. Corrosion may involve substantial material loss, but that is not universally true. Corrosion when manifested as pitting can cause very small total loss of material while destroying the component integrity. Similarly, hydrogen embrittlement can occur with no material loss, when hydrogen produced in an electrochemical fashion, enters the material leading to its loss of material properties. The study of corrosion therefore is also a study of material-environment interaction through a set of electrochemical reactions. It naturally requires detailed understanding of the metallurgy of the material including time dependent changes in the metallurgy, stresses present in the component, operational parameters, environment chemistry and reaction thermodynamics and kinetics. A point of view proposed by Professor Staehle, a well known corrosion expert, is that all engineering materials are reactive chemically and that the strength of materials depends totally upon the extent to which environments influence the reactivity and subsequent degradation of these materials. In order to define the strength of an engineering material for a corrosion based design it is essential to define the nature of the environments affecting the material over time. The corrosion event is a culmination of the conjoint action of various factors

Notch Effects in Corrosive Failure

Notches act as stress raisers in any application. There are a few other detrimental effects of notches when an environment is involved. For a simple corrosive environment, the notch chemistry and electrochemistry are significantly different from that of the bulk environs. When hydrogen embrittlement is a possible failure mode, the presence of notch has an added significance, in defining the hydrogen transport and distribution. This article discusses the notch effects in general as it pertains to corrosive failure and hydrogen embrittlement in specific. The comparative studies on different notch geometries reveal the importance of notch dimensions in the defining the extent and morphology of cracking in the presence of hydrogen. A rationalization has been attempted involving notch hydrogen availability

Electrical, Magnetic and Electrochemical Behaviour of Nanocrystalline Fe70.5Nb4.5Cu1Si16B8 Alloy

The electrical, magnetic and electrochemical behaviour of Fe70.5Nb..5CuiSi,638 has been studied in the as-received and heat treated conditions. The as-received material was amorphous which crystallized in two different stages at 780K and 940K when heated continuously. At the primary crystallization stage, nanometre sized grain of ordered FesoSi20 phase was formed. The superior soft magnetic properties were achieved after primary crystall-ization which were attributed to the averaging out of magnetocrystalline anisotropy due to the nanocrystalline structure and the reduction of magnetoelastic anisotropy energy due to the negative magnetosirictive nature of Fe8oSi20 phase and positive magnetosirictive value of the rest amorphous phase. After primary crystallization spon-taneous passivating nature of the alloy is also observed in electrochemical study

Influence of Quench Rates on the Properties of Rapidly Solidified FeNbCuSiB Alloy

FeNbCuSiB based materials were produced in the form of ribbons by rapid solidification techniques. The crystallization, magnetic, mechanical and corrosion behaviour were studied for the prepared materials as a function of quenching rate from liquid to the solid state. Higher quench rates produced a more amorphous structure exhibiting superior soft magnetic properties with improved corrosion resistance

Pitting Stochastic Study in Airframe Aluminium Alloy using Non-linear Ultrasonic

Pitting corrosion is considered to be one of the principal degradation mechanisms for high-strength aluminum alloys. The aircraft airframe has been the most demanding application for aluminum alloys. The combined effects of corrosion and cyclic loading have been shown to produce cracks from corrosion pits and pits have frequently been the source of cracks on aircraft components operating in fleets. Once the pit or group of pits form, the rate of pit growth is dependent mainly on the material, environmental conditions and type and state of stress. Therefore, to estimate the total corrosion fatigue life of a component, it is of great importance to develop realistic models to establish the component life in these situations and to formulate methods by which designers and operators know likely sources of pitting early in the design and fleet operation. There are certain gaps in knowledge with regards to life prediction for pitting initiated fatigue. The need is to gauge the extent of pitting damage of a component or material non-destructively and predict the remaining life through superimposition of the pertinent operational, environmental and material parameters. However, a foolproof non-destructive means to characterize and three-dimensionally map pits is not available. The pitting phenomenon has to be analyzed statistically and the kinetics of pitting assessed through a change in the statistical distribution parameter of pits rather than deterministic equations relating pit dimensions to time. In this work we have applied high frequency ultrasonic and non-linear ultrasonic to assess the damage due to pitting and attempt has been made to establish correlations between this non-destructive tools and pit stochastic

Fatigue crack growth retardation in an HSLA steel in benign environments

The crack growth and closure were examined for fatigue loading of an HSLA steel in non-corroding media. R and ΔK dependent significant crack growth retardation was observed in NaOH. Presence of a passive film at high R and self repair of the film and formation of an additional oxide layer at low R could explain the retardation

Effects of Cold Deformation Prior to Sensitization on Intergranular Stress Corrosion Cracking of Stainless Steel

The effects of deformation, prior to sensitization, on intergranular stress corrosion cracking (IGSCC) were studied on the AISI 304 (UNS S30400) stainless steel (SS). The degree of sensitization (DOS) was quantified by the double loop electrochemical potentiokinetic reactivation (DL-EPR) method. The susceptibility to IGSCC was investigated by the slow strain rate test (SSRT) carried out in polythionic acid (PTA) solutions. The results were complemented by scanning electron microscopy (SEM) fractographs. Deformation was found to accelerate sensitization, and a peak in sensitization vs. deformation was always observed. This peak was found to shift toward lower deformations with an increase in sensitization temperature. At 700°C, prior deformation is able to desensitize or heal the SS after 24 h. IGSCC was observed in AISI 304 SS after some treatments. No one-to-one correspondence was observed between IGSCC and DOS; this could be explained by the fact that the DOS measured by the DL-EPR indicates the depleted regions below ~15% Cr, whereas IGSCC depends on the availability of continuous grain boundary paths that are chromium-depleted, along with strain rate and environment (pH, temperature, etc.). Deformation prior to sensitization causes carbide formation and chromium depletion to occur near dislocations within the grain interiors, in addition to along grain boundaries. The DOS does not differentiate between these interior regions and the grain boundary regions, and shows Sensitization is a common phenomenon in stainless steels (SS) when they are exposed to temperatures ranging from about 400°C to 800°C.1-20 Classical sensitization results from the nucleation and growth of chromium carbide along grain boundaries (in solution-annealed SS and nickel alloys) with simultaneous depletion of chromium in adjacent grain boundary regions. The extent of chromium depletion in near grain boundary regions is limited by the equilibrium concentration of chromium at the carbide-matrix interface. The equilibrium chromium level depends on the temperature, the chromium activity coefficient, the carbon activity, and the equilibrium constant for carbide formation. Hall and Briant showed the equilibrium chromium concentrations to be 6.6, 8.4, and 10.8 wt% in AISI 316LN(1) (UNS S31603)(2) sensitized at 600, 650, and 700°C, respectively.21 Sensitization occurs in the temperature range where carbide is thermodynamically stable (500°C)