On-line corrosion monitoring in geothermal district heating systems. I. General corrosion rates

General corrosion rates in the geothermal district heating systems in Iceland are generally low, of the magnitude 1 μm/y. The reason is high pH (9.5), low-conductivity (200 μm/y) and negligible dissolved oxygen. The geothermal hot water is either used directly from source or to heat up cold ground water. The fluid naturally contains sulphide, which helps keeping the fluid oxygen-free but complicates the electrochemical environment. In this research on-line techniques for corrosion monitoring were tested and evaluated in this medium. Electrochemical methods worked well as long as frequency was kept low but ER worked better if oxygen was present.     

Localized corrosion behaviour of reinforcement steel in simulated concrete pore solution

The correlation of localized corrosion behavior and microstructure of reinforcement steel in simulated concrete pore solutions was investigated. The SEM/EDS analysis showed that most of ferrite, minor amount of pearlite and some MnS inclusions existed on the steel surface. The SKPFM results indicated a higher corrosion tendency at the ferrite grain boundaries, pearlite grains and MnS inclusions. The EIS and electrochemical polarization measurements demonstrated the influence of pH and chloride concentration on the corrosion behavior. In situ optical observations and AFM images revealed a detail of the localized corrosion behavior, which was in good agreement with the results from the other measurements.     

Localized corrosion of magnetite on ferritic–martensitic steels exposed to supercritical water

Magnetite usually forms on ferritic–martensitic (F–M) stainless steels as a protective barrier when the steels are exposed to supercritical water (SCW). However, a novel localized corrosion was observed on magnetite induced by adsorbed Cu₂S. The morphology and chemistry of the localized corrosion were studied by means of scanning electron microscopy, focused-ion beam, energy dispersive X-ray spectroscopy, and Auger electron spectroscopy. The mechanism of the development of the localized corrosion is elucidated in this paper. The presence of Cu₂S or related species should be eliminated for the applications of F–M stainless steels in SCW environment.     

Electrochemical evaluation of under-deposit corrosion and its inhibition using the wire beam electrode method

A new experimental method has been applied to evaluate under-deposit corrosion and its inhibition by means of an electrochemically integrated multi-electrode array, namely the wire beam electrode (WBE). Maps showing galvanic current and corrosion potential distributions were measured from a WBE surface that was partially covered by sand. Under-deposit corrosion did not occur during the exposure of the WBE to carbon dioxide saturated brine under ambient temperature. The introduction of corrosion inhibitor imidazoline and oxygen into the brine was found to significantly affect the patterns and rates of corrosion, leading to the initiation of under-deposit corrosion over the WBE.     

Observation of corrosion behavior of stainless steels in a salt manufacturing plant environment using the multichannel electrode method

Crevice corrosion of four kinds of stainless steel, SUS316L, NAS64, NAS185N and NAS254N, in saturated NaCl solution at temperatures up to 100 °C was investigated using the multichannel electrode method. In this method, a pile of five individual working electrodes (WEs) of stainless steel sheet were embedded in epoxy resin and a small hole penetrating through the five WEs was treated as an artificial crevice. Time transition and distribution of the coupling current between the five WEs were measured as a function of crevice depth, kind of stainless steel, temperature and concentration of dissolved oxygen (DO). Anodic or cathodic coupling current on the five WEs of SUS316L changed depending on their corroding state. On the other hand, NAS64, NAS185N and NAS254N showed that the WE outside the crevice contributed as a cathode and that WEs inside the crevice contributed as an anode. The coupling current on SUS316L was strongly affected by concentration of DO, while the coupling current on NAS64, NAS185N and NAS254N was not affected by DO, probably due to the establishment of a passive state inside the crevice.     

The effect of synthetic zinc corrosion products on corrosion of electrogalvanized steel. II. Zinc reactivity and galvanic coupling zinc/steel in presence of zinc corrosion products

The reactivity of zinc under synthetic zinc patinas and the galvanic coupling in steel/patina/Zn are studied. Zn₅(OH)₆(CO₃)₂ and Na₂Zn₃(CO₃)₄⋅3H₂O inhibit zinc anodic dissolution in NaCl, while Zn₅(OH)₈Cl₂ H₂O and Zn₄(OH)₆SO₄ nH₂O do not. The galvanic current in steel/patina/NaCl/Zn is smaller as compared to steel/NaCl/Zn. The inhibiting effect decreases with time for Na₂Zn₃(CO₃)₄⋅3H₂O or Zn₄(OH)₆SO₄ nH₂O due to the transformation into Zn(OH)₂. In NaHCO₃, the polarity between zinc and steel can reverse. The effect of confinement on the cathodic current is stronger than the initial effect of patina which is explained by the instability of the patinas under rapid pH-increase.     

Effect of cathodic protection on corrosion of pipeline steel under disbonded coating

In this work, a test rig was developed to study the effect of cathodic protection (CP) on corrosion of X70 pipeline steel in the crevice area under disbonded coating through the measurements of local potential, solution pH and dissolved oxygen concentration. Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.     

Influence of ultrasound on pitting corrosion and crevice corrosion of SUS304 stainless steel in chloride sodium aqueous solution

The change of polarization curves and surface morphologies of SUS304 stainless steel was investigated in 3.5 mass% NaCl solution with or without the application of ultrasound (US). As the result, both the pitting corrosion and the crevice corrosion were largely suppressed by the application of US. The reason is attributed to the decrease in the concentration of hydrogen and chloride ions in pits or in the crevice by removing the corrosion product and stirring the liquid there.     

Heat-transfer corrosion behaviour of cast Al alloy

Heat-transfer corrosion behaviour of an ISO 2379 cast Al alloy was studied in antifreeze radiator coolant under heat-rejecting condition. Extensive analyses of microstructures and corroded surfaces were carried out under the optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X-ray diffractometer. Heat-rejecting condition led to a cavitation process and cavities were observed within the α-Al matrix. Crevice corrosion was predominant at oxygen depleted regions in heat-transfer corrosion cell. Al₂Cu, Al₁₅(Fe,Mn)₃Si₂ dendrites, Al₄Cu₂Mg₈Si₇ and Si phases served as the effective cathodes resulting microgalvanic corrosion at the anodic site of α-Al matrix.     

Indoor atmospheric corrosion in Cuba. A report about indoor localized corrosion

Indoor weight loss of steel, chloride, sulphur compounds and dust deposition rate were determined in six storehouses having different characteristics. Relative humidity and temperature were determined in three storehouses. A model for indoor corrosion of steel depending on time of exposure and deposition of dust, sulphur compounds and chlorides is proposed. Dust deposition plays an important role indoors. The position of the sample has also a significant influence on corrosion. Indoor corrosion aggressivity in Cuban storehouses ranges in classification IC3 and IC4 according to the new ISO proposal of indoor aggressivity.A report about the presence of localized corrosion indoors (filiform like) using a special designed sample is made.     

Alteration of wettability and inhibition of corrosion in narrow aluminium 7075 gaps by thin polymer films

The admicellar polymerization process was adapted to deposit polystyrene (PS) and poly(methyl methacrylate) (PMMA) onto aluminium 7075 surfaces. Characterization of the films was performed using FTIR spectroscopy, scanning electron microscopy and atomic force microscopy. Dynamic contact angles confirmed the hydrophobic nature of modified aluminium substrates. Artificial crevices made from coated aluminium 7075 coupons were developed to simulate the crevices formed on aircraft. Water uptake measurements showed that PS delayed water penetration in the crevices for almost 4 h. Accelerated corrosion tests revealed the protective characteristics of PMMA. Image analysis was used to quantify the extent of corrosion in the crevice.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

Effect of biofilm on ennoblement and localized corrosion of stainless steel in fresh dam-water

AISI 304 stainless steel specimens were exposed in-situ to fresh dam-water for nearly 2 years. Open circuit potential (OCP) of the specimens becomes remarkably ennobled after exposure for about 40 days. The ennobled OCP of coupons is greatly affected by both immersion depth and sunlight. It is found that ammonium, nitrate, nitrite and chloride ions are enriched in the biofilm. It is suggested that the reduction of nitrate may be partly responsible for the OCP ennoblement. Localized corrosions at welds and crevices of specimens are attributed to both the OCP ennoblement of specimens and the chloride ion enrichment in the biofilm.     

The crevice corrosion behaviour of stainless steel in sodium chloride solution

The crevice corrosion behaviour of 13Cr stainless steel in NaCl solution was investigated mainly by electrochemical noise measurements, considering the influences of the crevice opening dimension (a) and the area ratio of the electrode outside the crevice to the one inside the crevice (r). Results show that the increase of r value prolongs the incubation period of crevice corrosion, but crevice corrosion develops rapidly once the crevice corrosion occurs. The crevice corrosion develops preferentially at the crevice bottom and then spreads to the whole electrode surface. Proton could reduce on the uncorroded area and hydrogen bubbles form inside the crevice.     

Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values

Reinforcement steel embedded in six different concrete mixes was exposed to chloride by wetting/drying cycles. Various parameters were continuously monitored during more than 1 year. Cement replacement with fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, by far the most dominant influencing parameter was the steel/concrete interface since corrosion initiated on the lower side of the rebar (with respect to casting direction) regardless of binder type and w/b ratio. In many cases, after the first signs of depassivation, a marked increase in chloride content was required to prevent repassivation and to enable stable pit growth.     

Pit to crack transition in stress corrosion cracking of a steam turbine disc steel

Long term exposure tests have been carried out on a 3 NiCrMoV steam turbine disc steel in the form of cylindrical tensile test specimens self-loaded to 90% of σ_0.2 and exposed to three environmental conditions, viz. deaerated pure water, aerated pure water, and aerated water containing 1.5 ppm of chloride ion. Pitting occurred in all environments but the density and depth of pits in the chloride-containing medium was markedly greater. No cracking was observed in deaerated pure water but cracks initiated in aerated water between 13 and 19 months and in less than 7 months in aerated 1.5 ppm Cl⁻ solution. The probability of a crack initiating from a pit of specific depth in aerated solution could be described well by a Weibull function. Profiling of pits and cracks in the disc steel tested in aerated 1.5 ppm Cl⁻ solution showed that there while there were many cracks with a depth greater than that of the corresponding pit the depth of some cracks was smaller than that of the corresponding pit, suggesting that cracks do not necessarily initiate from the bottom of the pits. The growth rate of short cracks emerging from pits appeared greater than that of long cracks in fracture mechanics specimens.     

Application of the multichannel electrode method to monitoring of corrosion of steel in an artificial crevice

Crevice corrosion of iron was evaluated using the multichannel electrode method in which 10 individual working electrodes (WEs) of pure iron were embedded in resin, placed in an artificial crevice in the range from 0.5 mm to 2.0 mm, and immersed in 0.51 mol dm⁻³ NaCl solution. The WEs were connected to an electronic circuit which allowed galvanic coupling between them and measurement of their individual coupling current or open circuit potential. Time-transient of the spatial distribution of coupling current and open circuit potential showed sequential transition of the coupling current on WEs at the middle position of the crevice from cathode to anode. The WE near the opening of the crevice initially showed a large anodic current, then a decreasing the anodic current corresponding to the current transition of other WEs, and finally a large cathodic current coupled with the other anodic WEs in the crevice. The transition of coupling current was explained by the change in pH and concentration of dissolved oxygen in the crevice. Thickness of the gap of the artificial crevice affected the transition behavior of coupling current distribution. For example, slower current transition with smaller coupling current was found in the case of a narrower gap. Such properties were related to the introduction and consumption of dissolved oxygen in the crevice solution and the circulation of gap solution from/to the outside of the crevice.     

Fe–C-coated fibre Bragg grating sensor for steel corrosion monitoring

A steel corrosion sensor based on fibre Bragg gratings (FBGs) has been developed and tested in this paper. The steel corrosion sensor has been formed by electroplating a Fe–C film on a metallized FBG. The coated FBG sensors have been tested in cabinets in which the availability of chloride ions and oxygen are controlled. The microstructural changes have been investigated by optical microscopy and SEM imaging. The FBG spectra are collected by a spectrometer. The results show that the spectra change, the wavelength shift, and the occurrence of multiple peaks are related to the corrosion degree of the Fe–C film.     

Pitting of zinc: Observations on atmospheric corrosion in tropical countries

A five-nation study has investigated the mechanisms and rates of the atmospheric corrosion of zinc and steel in tropical regions in Australia, Thailand, Indonesia, Vietnam and The Philippines. For the study, 18 exposure sites encompassing severe marine, marine, severe industrial, industrial, marine/industrial, urban and highland environments were established across the countries. At each location, zinc and steel plates were exposed for periods of three months and one year, and measurements were taken of a wide range of surface-response and climatic parameters, including gaseous SO_x and NO_x, airborne salinity, relative humidity (RH) and temperature, rainwater composition, surface temperature and time of wetness (TOW). Exposed plates were used to determine mass loss, the nature of corrosion products (using FTIR and SEM-EDS) and the morphologies of corrosion layers (via SEM-EDS). Regression analysis indicated that the prime factors controlling zinc corrosion rate were climate (temperature and rainfall) and surface-response (TOW), and surprisingly not pollutant levels, despite significant variation in SO_x levels across the sites. SEM studies indicated the presence of pitting below the oxide layers on zinc, particularly those plates exposed at marine and other sites with relatively low SO_x levels. In contrast, no pitting was observed (or pits had very low aspect ratios) in the specimens exposed at sites with high SO_x levels. The possible processes leading to the observed damage patterns are discussed.