The application of an electrochemical technique to determine the porosity of electroless nickel coatings produced in hypophosphite baths

An electrochemical polarisation technique was employed to measure the porosity of electroless nickel (EN) coating. The technique is based on the change observed in the electrochemical parameters with varying cathode and/or anode area on a bimetallic corroding surface. The nickel coating test samples were obtained from a hypophosphite plating bath in the presence of different complexing agents. This technique was used to estimate the effect of coating thickness on porosity and the influence of addition of different complexing agents to EN baths on porosity. The results suggest that, unlike other conventional methods, the electrochemical, a non-destructive method, can detect the smallest pore in an EN-coating and quantify its size in terms of pore area fraction

Corrosion evaluation and monitoring practices

Corrosion tests are aimed at to evaluate and select suitable materials for process, evaluation of corrosivity of environment, and to check the effectiveness of the applied corrosion control measures. These tests may be performed in laboratory, in actual process plant, or in the field such as atmosphere tests. The laboratory tests are generally carried out using small specimens, small volume of solution (simulated synthetic or from actual process). These are useful and economic to screen out the most useful or useless materials for the process orto test further in plant environment. The actual in-plant test needs plant availability for the desired test and may take certain time and cost. There are several methods being used for evaluating corrosion. Laboratory test methods may be categorized as an electrochemical, which are based on the measurements of current evolves from electrochemical reactions, while non-electrochemical techniques are based on the overall weight lost by the specimen or surface features due to corrosion such as pit, crevice, crack or microstructural changes appeared after the test. However, for choosing appropriate method one needs to know about the type of corrosion against which resistance of materials is being sought. For instances, to determine the thinning rate of a steel pipe, methodologies for general/uniform corrosion such as weight loss, polarization resistance etc. will be more useful than the others. The knowledge of a parameter of concern that defines the resistance of materials against specific form of corrosion is also important. For example, pitting and protection potentials (obtained in electrochemical experiments) are characteristic features (parameters of concern) to illustrate the pitting susceptibility of any passivating alloy in a corrosive environment. Several methods being practiced to evaluate different types of corrosion are discussed in forthcoming sections

Different Forms of Corrosion in Industries

Corrosion is an inevitable process which causes degra-dation of metals/ alloys when exposed to the environment. It causes huge economic losses in terms of maintenance and repair cost, unplanned shutdowns of plant, loss of costly products due to leakage. Besides this, aesthetic loss, contamination of the product, safety and human loss are important. According to a survey conducted by federal highway authority (FHWA) of USA and MACE, direct corrosion costs were estimated to be about 137 billion dollars annually due to all sectors in United States of America. Corrosion losses are roughly approximated to be about 1.5 lakhs crore due to various sectors in India

2-Methylbenzothiazole Complexes of Hg(II) Halides & Pseudohalides

1022-102

Impact of Filtrate Recycling on Corrosion of Stainless Steels in Hypochlorite Solutions

To reduce the pollution load, industries using chlorine and its compounds are adopting filtrate recycling as partofclosed-loop concept This strategy, however, enhances the corrosivity of the process liquors. To cope this problem by understanding corrosion process prevailing in the system, detailed corrosion experiments were conducted with stainless steels AISI grade 304 and 315 (hitherto used material) in hypochlorite solutions that fails to withstand the corrosivity of the recycled liquours. The mechanism of the reaction is also discussed

Methylpyrazine Complexes of Ag(I), Zn(II) & Cd(II) Nitrates

562-56

Ceramic coatings by sol-gel method for corrosion protection of mild steel

Mild steel finds extensive applications in automobile, household appliances, heavy construction and business machines due to its mechanical properties and machinability at low cost. However, it is prone to corrosion that leads to industrial accidents and loss of material resources. Sol-gel derived ceramic coatings have been deposited on mild steel (MS) with a prior conversion coating for its corrosion protection. The prior conversion coating led to the formation of well adherent coatings of Al2O 3 , ZrO 2 and TiO 2 . The Scanning Electron Microscope (SEM), X-ray diffractometer (XRD) and FT-IR was used to study the morphology, phases formed and nature of bonding in the coating. The electrochemical corrosion behavior of the coated MS was evaluated in 3.5 wt.% NaCl solution. The coatings show barrier type protection of MS up to about 1 V SCE . The Al 2 O 3 and ZrO 2 coatings were observed unaffected during prolong immersion in 3.5 wt.% NaCl solution. The physicochemical and electrochemical properties have been correlated and discussed

Response of Weld Joints in Stainless Steel 304LN Pipe to Low Temperature Sensitization

Heat affected zones adjacent to the welds are generally considered sensitive locations for failure of heat transport system pipelines (made of austenitic stainless steels) in nuclear power industries. Present study includes understanding the sensitization and IGC behavior of nuclear grade stainless steels (AISI 304LN) at low temperatures. Two different solution annealed (pipes) AISI 304LN and weld joints were exposed to 400 and 450°C for 125-8000 hours followed by furnace cooling. Sensitization was quantified in terms of degree of sensitization (DOS) by using double-loop electrochemical potentioldnetic reactivation (DL-EPR) method. The oxalic acid (in 10% oxalic acid solution) etching and copper-copper sulfate-sulfuric acid test as per the ASTM 262 Practice 'A' and 'E' were performed to detect and quantify the susceptibility to intergranular corrosion. Specimens from two different pipes showed variations in susceptibility to sensitization and IGC. Specimens designated, as 304LN-P1 was much less prone to sensitization as compared to 304LN-P2. While HAZ of former showed `step' type structure the other one (304LN-P2) produced mostly 'dual' and `step', in a few cases, during oxalic acid etching. While bending after boiling in copper-copper sulfate- sulfuric acid solution, certain specific time/ temperature combinations produced fissures or even large cracks on the weld pool of both stainless steels

Electrochemical and mechanical behavior of laser processed Ti–6Al–4V surface in Ringer’s physiological solution

Laser surface modification of Ti–6Al–4V with an existing calcium phosphate coating has been conducted to enhance the surface properties. The electrochemical and mechanical behaviors of calcium phosphate deposited on a Ti–6Al–4V surface and remelted using a Nd:YAG laser at varying laser power densities (25–50 W/mm 2 ) have been studied and the results are presented. The electrochemical properties of the modified surfaces in Ringer’s physiolog- ical solution were evaluated by employing both potentio- dynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The potentiodynamic polar- izations showed an increase in the passive current density of Ti–6Al–4V after laser modification at power densities up to 35 W/mm 2 , after which it exhibited a decrease. A reduction in the passive current density (by more than an order) was observed with an increase in the laser power density from 25 to 50 W/mm 2 . EIS studies at the open circuit potential (OCP) and in the passive region at 1.19 V showed that the polarization resistance increased from 8.274 9 10 3 to 4.38 9 10 5 X cm 2 with increasing laser power densities. However, the magnitudes remain lower than that of the untreated Ti–6Al–4V at OCP. The average hardness and modulus of the laser treated Ti–6Al–4V, evaluated by the nanoindentation method, were deter- mined to be 5.4–6.5 GPa (with scatter \±0.976 GPa) and 124–155 GPa (with scatter \±13 GPa) respectively. The corresponding hardness and modulus of untreated Ti–6Al–4V were *4.1 (±0.62) and *148 (±7) GPa respec- tively. Laser processing at power densities [35 W/mm 2 enhanced the surface properties (as passive current density is reduced) so that the materials may be suitable for the bio-medical applications

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)