Microbially influenced corrosion of stainless steel by manganese oxidizing microorganisms

Based on the corrosion behaviour of stainless steels in fresh water and on the electrochemical properties of higher manganese oxides, the mechanism “Microbially influenced corrosion by manganese oxidizing microorganisms” (MIC by MOMOs) is presented as the consequence of biomineralized manganese oxides in contact with the metal. Localized corrosion may develop at elevated but normally undercritical chloride concentration in the water. The mechanism was found useful in the analysis of certain cases of unexpected failure of stainless steel in fresh water.     

Advanced studies on the influence of manganese on the potential increase of stainless steels

The influence of manganese on the time course and on the final value of the open‐circuit potential of microbially influenced stainless steel (SS) has been examined by variation of the concentration of manganese in the aqueous phase and on the surface of the metal sample. Under the influence of bacteria stable final values of the open‐circuit potential are attained at 540 mV or 620 mV depending on the manganese concentration in the aqueous phase, the duration of the experiment, and the supply of oxygen. These potential values can also be fixed electrochemically by anodic deposition of MnO₂ on the metal surface. According to the results obtained with different techniques, the electron flow from the metal towards the oxygen is a combination of biotic and abiotic processes. Besides the microbial oxidation of manganese the electron flow via a Mn‐based redox enzyme system has to be discussed.     

Rainbow fringes around crevice corrosion formed on stainless steel AISI 316 after ennoblement in seawater

The crevice corrosion occurrence probability of stainless steel (SS) AISI 316 was increased under ennoblement condition due to chemically added H₂O₂ into seawater. The H₂O₂ was used to simulate the important factor causing ennoblement in natural marine biofilm. Morphology of the crevice corrosion was observed using an incident‐light source microscopy. Some interesting “rainbow” fringes were observed around micro‐crevices. The mechanism was discussed from the ions diffusion and potential distribution during the crevice formation. This result shows that under ennoblement condition the colored fringe is a distinct characteristic of the morphology of localized corrosion for stainless steel.     

Influence of dissolved oxygen content on the bacteria‐induced ennoblement of stainless steels in seawater and its consequence on the localized corrosion risk

The ennoblement of stainless steel (e.g., the increase of open circuit potential [OCP]) is associated with bacterial colonization. This increases the risk of localized corrosion as the critical pitting/crevice potential can be overcome, especially for lower grade stainless steel. In this study, we assessed the influence of dissolved oxygen content (DOC) on the crevice corrosion of duplex and super duplex stainless steels. In addition, we used DNA amplicon sequencing to identify the bacteria most likely associated with the ennoblement. Above approximately 100 parts per billion (ppb) of dissolved oxygen, the ennoblement of OCP was observed leading to an increased risk of localized corrosion. Below approximately 100 ppb of dissolved oxygen, no ennoblement occurred and the risk of localized corrosion was reduced. We identified certain hydrocarbon‐degrading bacteria whose presence correlated with the ennoblement of super duplex stainless steel at saturated DOC. The role of these bacteria is not clear yet, but their distribution indicates a possible involvement in stainless steel ennoblement in seawater.     

2209和2507双相不锈钢堆焊层组织结构及耐蚀性对比

利用光学显微镜、X射线衍射仪、扫描电子显微镜和能谱仪等,对2209和2507双相不锈钢FCAW堆焊层焊态、退火态的组织结构和耐蚀性进行对比研究。结果表明：两种堆焊层焊态时均主要由铁素体α和魏氏体状奥氏体γ组成,在α晶内及α/γ界面处存在少量细条状的二次奥氏体γ2,在α/γ、α/α界面处还存在极少量的点状σ相;两种堆焊层经690℃×8h退火后,σ相和γ2均明显增多。两种堆焊层焊态的耐蚀性均合格,且2507型堆焊层的耐蚀性优于2209型堆焊层;而两种堆焊层退火态的耐蚀性均急剧下降、严重不合格,这主要是由于退火过程中堆焊层内析出大量σ相和γ2所致。     

Two‐step electrosynthesis of polypyrrole for corrosion protection of stainless steel

Smooth polypyrrole (PPy) films were successfully electrosynthesized on a stainless steel (SS, 1Cr18Ni9) surface by a self‐catalytic two‐step process. SS substrate dissolution during PPy electrosynthesis was effectively depressed. The redox properties and corrosion behavior of PPy film coated SS electrodes were investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. The reduction of PPy was found to include two reaction processes: an anion dedoping process and a cation insertion process. The PPy film was found to lose its electroactivity when polarized at high potentials due to peroxidation. EIS of SS/PPy mainly corresponds to PPy film response and charge transfer resistance decreases as film thickness increases, indicating that SS corrosion can be inhibited effectively by the PPy film.     

1‐Methyl‐3‐pyridine‐2‐yl‐thiourea as inhibitor for acid corrosion of stainless steel

The inhibiting effect of 1‐methyl‐3‐pyridine‐2‐yl‐thiourea (MPT) on the corrosion of stainless steel in 0.5 M H₂SO₄ solution was investigated using weight loss and potentiostatic polarization techniques. The percentage inhibition efficiencies and surface coverage degrees increased with increasing additive concentration. Potentiostatic polarization studies revealed that MPT is of the mixed‐type inhibitor in 0.5 M H₂SO₄ solution. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25–55 °C E_a, ΔH_a and possible mechanism have been proposed. Scanning electron microscopy (SEM) was done from the surface of the exposed sample indicating uniform film on the surface.