Thick Corrosion Product Research Articles

Influence of chloride, sulfate and bicarbonate anions on the corrosion behavior of AZ31 magnesium alloy

Influence of chloride, sulfate and bicarbonate anions on the corrosion behavior of AZ31 magnesium alloys in dilute solutions was investigated by electrochemical measurements, whereby corrosion maps in terms of electrode potential and solution concentration were obtained. AZ31 alloy exhibited corrosion and passivation zones in those dilute solutions. The passivation zone was strongly influenced by the kinds of the solution used and the broadest zone was obtained in Na 2SO 4, followed by NaCl and NaHCO 3. AZ31 alloy had a better corrosion resistance in Na 2SO 4, attributing to the lowest attack by SO 4 2− and a thick corrosion product covering the surface which inhibited the corrosion attack. However, despite a much lower passivation zone in NaHCO 3 as compared to the other two solutions, the corrosion rate in NaHCO 3 was found to be the lowest. The values of open-circuit potential were in the passivation zone when [Cl −] was less than 0.2 mol/L in NaCl, and [SO 4 2−] and [HCO 3 −] were both less than 0.3 mol/L in Na 2SO 4 and NaHCO 3. In the passivation zone, the formation of surface films or corrosion products on the material surface retarded further corrosion in dilute solutions. In the corrosion zone, pits grew continuously in depth and diameter in NaCl and Na 2SO 4 resulting in the localized corrosion through the agglomeration of pits. The formation of crystalline-like MgCO 3 products exhibited the general corrosion in NaHCO 3.

A corrosion study of the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes techniques

In this study, long term corrosion mechanisms are approached through the characterisation of corrosion products formed on ancient artefacts over 500 years. Thirty-one artefacts were sampled in the Amiens cathedral. The thick corrosion product layers have been characterised at a microscopic scale by coupling complementary microbeam analytical techniques (SEM-EDS, Raman microspectroscopy, X-ray microdiffraction and microfluorescence, X-ray absorption microspectroscopy under synchrotron radiation). The main phase of the corrosion layer is goethite but lepidocrocite and akaganeite are also present locally in the corrosion layer. In addition, the presence of low crystallinity phases (feroxyhyte and/or ferrihydrite) is showed. These phases are electrochemically reactive, thus they could play a key role in the corrosion mechanisms.

Accelerated low water corrosion of carbon steel in the presence of a biofilm harbouring sulphate-reducing and sulphur-oxidising bacteria recovered from a marine sediment

Investigations were undertaken to elucidate causes of accelerated low water corrosion (ALWC) of steel piling in a harbour in Southern England. Visual inspection revealed features characteristic of ALWC such as the presence of poorly adherent, thick corrosion products of varying morphology, often seen as large blisters randomly located on sections of the structure at the low water mark. Upon the removal of blisters, a bright surface covered with shallow pits was exposed. Representative samples of the corrosion products were collected from the structure and water and sediment specimens were retrieved from selected areas in the harbour for microbiological, chemical and microscopy testing. In the laboratory, field samples were enriched to detect and enumerate communities of sulphur-oxidising bacteria (SOB) and sulphate-reducing bacteria (SRB). Biofilms, comprising SRB and SOB populations isolated from a sediment sample were grown under static conditions on surfaces of electrodes manufactured from steel piling material. Linear polarisation resistance (LPR) measurements revealed that the corrosion rate of steel with biofilms (0.518 mm y −1) was higher than that recorded in sterile seawater alone (0.054 mm y −1) and in sterile seawater to which nutrient was added (0.218 mm y −1). Scanning electron microscopy (SEM) imaging demonstrated enhanced pitting under biofilms. The results of our investigation revealed for the first time that the attack on steel piling in the presence of sediment SRB and SOB populations was characteristic of ALWC.

Investigation of atmospheric corrosion of Zn using ac impedance and differential pressure meter

The proportional constant k of the Stern and Geary equation, for zinc corrosion in near-neutral chloride solution, has been experimentally estimated using i corr values determined from differential pressure and weight loss methods. In the initial stage, the k value has been found to be 13 mV. However, when the surface was covered with a thick corrosion product, the k value decreased down to 4 mV, and then maintained a constant value. The k value estimated from the weight loss method agreed with the one estimated from the differential pressure meter (DPM). Experimentally, it was revealed that a 10 mHz impedance could not be taken as the polarization resistance if the surface is covered with a thick corrosion product. However, a proportional relation between corrosion rate and reciprocal of impedance at 10 mHz ( Z 10 mHz −1 ) still exists, even though the surface is covered with a thick corrosion product.

Relationship of corrosion product films and recrystallization of 70Cu-30Ni alloy

The corrosion and microstructure of 70Cu-30Ni alloy plates and commercial tubes were investigated by processing, electrochemical technique and seawater exposure as well as optical microscopy, SEM, EDX and AES. The alloy plate with incomplete recrystallization, consisting of deformed and recrystallized grains, demonstrates loose and porous outer layers and denickelification in the inner layers for its corrosion product films, whereas the recrystallized alloy plate forms smooth and compact corrosion product films rich in nickel after immersion in seawater for 3 and 6 months. The incompletely recrystallized alloy tube keeps an active state and the recrystallized tube has a passive zone in the anodic polarization curve in 3.5% sodium chloride. Furthermore, the former shows loose and thick corrosion product films, which are of denickelification in the inner layer and contain a significant amount of seawater species in the outer layer, and on the surface of the latter are thin uniform and compact corrosion product films containing a Ni enrichment zone which becomes richer in Ni and wider with increasing immersion time. It is found that intergranular corrosion proceeds along the boundaries of deformed and recrystallized grains, and it is ascribed to electrochemical cells built for the reason of potential difference between the two kinds of grains.

The roles of tantalum and phosphorus in the corrosion behavior of Ni-Ta-P alloys in 12 M HCl

The corrosion behavior of Ni-10Ta-20P, Ni-10Ta and Ni-20P alloys in 12 M HCl solution open to air at 30 °C was investigated in order to clarify the roles of tantalum and phosphorus. Additions of 10 at % tantalum and 20 at % phosphorus to the nickel alloy result in excellent corrosion resistance and in the accumulation of tantalum and phosphorus in the surface film on this alloy owing to dissolution of the nickel from just after immersion, the highly protective passive film formed being stable during prolonged immersion. By contrast, the binary nickel alloy with only a tantalum addition is not passivated and dissolves actively, forming a thick corrosion product film. The nickel alloy containing only phosphorus is not corrosion-resistant, forming a nickel phosphate film, although the film is more stable than that formed on the nickel alloy containing only tantalum.

塩化ナトリウム水溶液中における各種炭化物の腐食挙動

The corrosion behavior of carbides has been studied by an electrochemical method in sodium chloride solution at 50°C. The anodic dissolution current of tungsten carbide and vanadium carbide readily increased with a little polarization from immersion potential. Tungsten carbide formed a thick corrosion product (WO3·2H2O). The corrosion product of vanadium carbide was slight by reason that the rate of its dissolution probably exceeded the rate of formation of oxide. Rapid anodic dissolution of titanium carbide and chromium carbide begun at about 0.5V (vs. Ag/AgCl), in the trans-passive region. Niobuim carbide was stable in sodium chloride solution.

Alternate Immersion Testing of Coated Sheet Steel

Abstract Alternate immersion cyclic exposure of prime coated and scribed steel sheet resulted in a lifting of the coating at the edges of the scribe. A thick oxide corrosion product film grew in the scribe and under the adjacent coating. Continuous immersion did not produce thick oxide films or a lifting of the coating. A zinc phosphate pretreatment limited the growth of the oxide to an area within the scribe and prevented attack at the coating/metal interface during cyclic exposures. Polarization resistance, which was measured during the immersion segment of the cycle, was greater for phosphated specimens, reflecting the lesser extent of corrosion at the scribe edges. However, sensitivity was not high enough to produce any advantage over simple visual examination and measurement. In the presence of thick oxide films in the scribe, potential changes were very slow, and a modified procedure was necessary for measuring polarization resistance. Galvanized and Zincrometal treated specimens were resistant to t...

The fractography of stress corrosion cracking in carbon steels

The stress corrosion cracking of carbon steels in a number of environments (nitrates, hydroxides, carbonates, chlorides and liquid ammonia) has been studied by scanning electron microscopy. The crack path depends on both heat treatment and environmental conditions. In annealed steels it appears that solutions which produce thick corrosion products induce intergranular cracking, while in solutions where adsorption induced inhibition occurs or thin films form, cracking is transgranular. Although the available fractographic evidence is ambiguous, it appears to rule out either hydrogen embrittlement or brittle films as being important in crack propagation. However, the fracture surface morphologies are consistent with the cracks propagating by anodic dissolution, possibly aided by mechanical tearing.

Effect of Metal Composition and Structure On Corrosion and Oxidation

Abstract Metallurgical factors such as heat treatment and cold-work are not important when corrosion in aqueous media is controlled by diffusion of reactants, e.g. dissolved O2, from the environment. They become important when corrosion control is focused at the metal surface, e.g. steel in acids. In this instance, galvanic cells depend less on where or how metal atoms are placed in their lattice, and more on compositional differences along the metal surface produced by heat treatment or plastic deformation. At elevated temperatures, compositional gradients within a metal are less important. The rate depends instead on diffusion through thick corrosion product layers and on the tendency for spoiling to occur during temperature fluctuations. Metallurgical factors assume greatest importance during initial oxidation, or for thin film formation.