Anodic pickling of stainless steels in sulphuric acid

Potentiostatic pickling in 20% aqueous H₂SO₄ was investigated on hot- and cold-rolled strips of stainless steels from industrial lines, in order to assess the potentiality of this technique. Experimental evidences for AISI Types 202, 304, 316, 410 and 430 are given. Optimum pickling conditions were defined on the basis of the potentiodynamic behaviour of oxidized specimens. Visual and electrochemical evaluation of pickled surfaces showed that:

1. The potentiostatic technique makes descaling and pickling of stainless steels quite feasible in sulphuric baths;
2. remarkable advantages result over ordinary chemical treatments in terms of speed of scale removal;
3. a higher efficiency of restoration of passivity properties also results.

     

Corrosion behaviour of innovative stainless steels in mortar

The main problem of the building sector is the limited durability of reinforcing concrete structures under highly aggressive environments due to the corrosion of reinforcements. In order to demonstrate that stainless steels are the adequate solution to avoid corrosion of reinforced concrete structures by contained chlorides, the corrosion behaviour of AISI 304 and AISI 316L stainless steels in mortar with two chloride doses is compared with the corrosion behaviour of three new stainless steels of low Ni content and reduced cost, and with the traditional carbon steel. The lowest chloride contamination (2% Cl⁻) has been used in this research to provoke corrosion in an active state of reinforcing carbon steel, whereas the highest one (5% Cl⁻) exceeds all expected levels found in the natural environment, including sea water. The new stainless steels remain in the passive state in mortar specimens with both chloride contents.     

腐植酸专用液肥对哈密市园林植物抗干热风的影响

简要介绍哈密市园林树木遭受干热风危害情况，腐植酸专用液肥是以黄腐植酸为主要成分，具有旱地龙的抗旱功能，初步对比试验，喷施腐植酸专用液肥能够减轻干热风对白蜡树、圆冠榆树的危害，结合正常的、科学的林木抚育管理，减轻干热风带来的经济损失，美化城市，美化家园。     

Corrosion of stainless steels covered by exopolymers

After a biologically induced corrosion in a 304 stainless steel tube electrochemical tests were carried out on the affected areas to characterise the nature of attack of the steel covered by the excreted exopolymers. Once the corrosive attack has been developed, the samples were removed from the solution, dried out for 1 month and re-immersed in the same well water which caused the attack. Polarization curves and impedance measurements made on non attacked welds, reference stainless steel and the metal covered by the exopolymer confirm that a reactivation of the corrosion process in previously damaged welded areas is produced. However, the corrosive attack has not been so severe than the initially produced by bacteria and after 28 days of immersion the progress of the attack is probably due to the previous activation of the steel surface rather than the microbiological activity.     

不锈钢在湿法磷酸中的应用

介绍湿法磷酸生产中杂质、操作条件、酸浓度、温度、搅拌、有饥物等对腐蚀的影响和材料选择的基本要素，并列出多种不锈钢的主要化学成份和几种工艺流程的材料选用表。     

抗菌不锈钢研究现状

综述了近几年国内外各种抗菌不锈钢的研究进展。介绍了复合抗菌不锈钢、涂层抗菌不锈钢、表面改性抗菌不锈钢以及合金型抗菌不锈钢的抗菌原理、特点、制造工艺,并展望了其应用前景。     

The service of the hearths of arc furnaces for melting stainless steels with the use of oxygen

Conclusions During the operation of arc furnaces specializing in the melting of stainless steels, chemical and phase changes occur in the magnesite hearths which lead to the formation of a single type structure in the monolith of rammed layer and upper courses of the structure.Petrographic studies showed that the monolith was formed as a result of phase inversions of the minerals of the hearth under the influence of the silicate constituents of the slag being produced, which partly settles to the bottom after discharge of the melt. These inversions greatly affect the physicochemical properties of the hearth and especially the refractoriness. The inversions which increase the refractoriness include the process of recrystallization of the periclase and chrome-spinel with the formation of strong and dense periclase-spinel growths in the upper layers of the hearth.The reduction in the refractoriness occurs owing to the formation of montichellite under the influence of calcium slag constituents and the gradual replacement of the forsterite by this silicate, and also the partial solution of periclase in the silicate melt.The thermal resistance of the hearth is favorably affected by the fact that the new silicate formations are distributed irregularly over the height of the hearth, and the nature of the distribution depends on their fluidity and temperature gradient in the hearth. In view of this consistency, the most refractory silicates, such as forsterite and mervenite, are localized in the upper strata of the hearth, and the montichellite, being the most fusible silicate, migrates to the lower layers.The result of all the processes and phase inversions taking place in the hearth is that the refractoriness of the most important layers of hearth (upper and middle) remains high during service. The reduction in refractoriness in the lower layer cannot greatly affect the life of the hearth since this layer is quite far away from the working region of the furnace and is in the low-temperature region.Large amounts of metals are detected only in the part of the hearth located in direct contact with the furnace bath; metal does not take a large part in the processes occurring in the hearth.The positive influence of the additions of chromite on the life of the hearth of electric furnaces determined during operation, can be explained by the fact that the chromite is essential for the formation of refractory chromespinel phases [2], contributes to the formation of periclase-spinel growths in the upper parts of the hearth, and accelerates the process of removing the less refractory silicates from the upper layers of the hearth to the lower [3].     

不锈钢在湿法磷酸中腐蚀行为研究

本文采用腐蚀实验和电化学测试方法对不锈钢在湿法磷酸中的腐蚀行为进行了研究，并进行钝化膜的组成与结构分析。研究表明，磷酸中加入ＳＯ４＾２＾－，ｃＬ＾－，Ｆ＾－均提高合金的维钝电流密度和致纯电流浓度，加速材料的腐蚀，这些杂质共存时腐蚀性更大。湿法磷酸中存在的Ｓｉ＾４＾＋，Ａｌ＾３＾＋，Ｍｇ＾２＾＋和Ｆｅ＾３＾＋起缓蚀作用。不锈钢纯化膜主要由Ｃｒ和Ｆｅ的氧化物组成，表面形成致密稳定的Ｃｒ２Ｏ３保护膜是提     

Corrosion behaviour of some stainless steels in chlorinated Gulf seawater

The corrosion resistance of conventional and high alloy stainless steels (specifically 316L, 317L, 904L, duplex 2205, 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565) was determined electrochemically in chlorinated and unchlorinated Arabian Gulf Seawater at 25 and 50 °C. The stainless steels 316L and 317L were used as reference alloys. The electrochemical potentiodynamic cyclic polarization method was used to determine the passive film break down potential (E _b), protection potential (E _prot) and maximum current attained on scan reversal (I _max). It was found that at 25 °C in chlorinated and unchlorinated seawater and at 50 °C in unchlorinated seawater, stainless steels 316L and 317L have poor resistance to corrosion, Stainless steels 904L and duplex 2205 at 25 °C in chlorinated and unchlorinated seawater showed good resistance to corrosion but at 50 °C these steels failed to resist. The high alloy stainless steels such as 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565 showed better corrosion resistance under all the test conditions.     

Comparative investigation of stainless steels used in drinking water distribution systems

Five stainless steel alloys were tested and compared to find a test method for the qualification of stainless steel used in drinking water systems. Determination of the pitting/critical potentials from the conventional cyclic polarization results was shown to be difficult when stainless steels have high Mo and Cr content. The electrochemical impedance data indicated that results received with the conventional methods do not necessarily reflect the steady state. The low frequency time constant determined from the impedance measurements showed that the anodic polarization should be conducted with very slow scan rate to get information about the mechanism. The steady state investigations were compared with the conventional electrochemical results to propose an electrochemical-based route as a standard qualification method. The qualification method was tested to be valid by a European round-robin-test.     

Self-organisation of nanoscaled pores in anodic oxide overlayer on stainless steels

The nanoscaled morphology of the overlayer covering stainless steels after electropolishing in perchloric acid-based electrolyte was explored mainly by AFM and SEM. Two kinds of stainless steels were tested. For the austenitic one (AISI 304L), a quasi-periodic arrangement of pores in this overlayer has been observed. Depending on the experimental conditions, the distance between neighbouring pores ranged from 20 nm up to 230 nm. This inter-pore distance varied either with the applied voltage or with the current density for a constant voltage. From XPS spectra performed on the nanostructured surfaces, analysis of the energy shifts of Cr and Fe 2p levels showed that the anodic overlayer was enriched in Cr atoms compared to the 304L steel bulk composition. For the austeno-ferritic duplex stainless steel, the electropolished surface exhibited nanoscaled pores, which had grown and self-organised on both phases but with different characteristic dimensions.     

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.     

Determination of critical pitting potentials of stainless steels in aqueous chloride environments

A new method for determining the critical pitting potentials, E_c, of stainless steels in aqueous chloride environments has been developed. The technique has the advantage of defining a discrete value of E_c which is dependent only on the composition and structure of the metal and the test environment. The irreproducibility of E_c measurements frequently discussed in corrosion literature is readily explained on the basis of these new measurements. Experimental data are presented for Fe-Cr and Fe-Cr-Mo alloys in a hot sea-water environment. These data suggest that the passivity of the alloys is attributable to an adsorbed layer on the metal surface, and that further film growth merely influences the induction time for pit nucleation.     

New findings on intergranular corrosion mechanism of stabilized stainless steels

Number of different sets of stabilized both ferritic and austenitic stainless steels with various alloying elements were evaluated to verify new findings on the intergranular corrosion mechanism. The intergranular segregation and precipitation were analyzed by using a transmission electron microscopy with an energy dispersive X-ray spectroscopy and a laser assisted three-dimensional atom probe. On the basis of the current result, it is newly proposed that the intergranular corrosion occurring in the stabilized both ferritic and austenitic stainless steels is induced by Cr-depletion due to segregation of un-reacted Cr atoms around carbides of stabilizer elements (Ti or Nb) along the grain boundary, but not due to formation of Cr-carbide. A prevention method for this type of intergranular corrosion is also suggested after critical evaluation on the effect of Cr, C, and Ni.     

Unified approach to localized and general corrosion of stainless steels

Localized and general corrosion of stainless steels is considered. In the case of localized corrosion, statistical parameters are deduced from the comparison of the distributions of breakdown and corrosion potential values to rank different materials, environments and designs. Cumulative frequency F is considered for breakdown (E_b) and corrosion (E_c) potential, and both are represented on a logarithm plot as E=E(log F). An extrapolated value of F (the risk H) corresponding to E_b=E_c is related to the field behavior of the material. The risk H is formally related to the corrosion current density (i) of the general corrosion approach. An analogy between the formalism of the probabilistic treatment of localized corrosion of stainless steels and that of uniform corrosion is highlighted through Monte Carlo simulation. Introducing a discretization model of the general corrosion, the time evolution of the surface roughness during general corrosion of stainless steels is reproduced in agreement with the experimental results. According to this model, the general corrosion can be treated on a similar basis of the localized corrosion in terms of discretization of corroding areas, and vice-versa localized corrosion can be treated on a similar basis of the general corrosion in terms of statistical distributions of the potential values.     

Structural evolution during mechanical alloying of stainless steels under nitrogen

Mechanical alloying of Fe-based powders under a nitrogen atmosphere has been recently converted into the subject of many researches. This paper reviews aspects of mechanical alloying of stainless steels under a nitrogen atmosphere. Nitrogen incorporation from the viewpoints of infusion mechanism, supersaturation, and kinetics is considered. The structural refinement, leading to nanocrystallization, and phase transformations, consisting of austenitization and amorphization, are outlined. Particularly, it is noted that nitrogen intensifies grain refinement and proceeds ferrite-to-austenite phase transformation and amorphization during mechanical alloying.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

In situ corrosion study of stainless steels in an MSF desalting plant

The corrosion resistance of different stainless steel grades have been studied as regards localized attack under service conditions. Test racks with welded samples with artificially applied crevices were inserted in three different flash chambers. Materials used for splash plates, brine piping and. condenser tubes were also tested.Beinge a pilot plant, various process conditions were tested. Hence, the materials have been exposed to different corrosive conditions, including stagnant seawater during shut-down periods.The results show that stainless steels of type 316L and higher alloyed exhibit good corrosion resistance, even in brine at high temperature viz pipes from the brine heater.     

Corrosion resistance of stainless steels in chloride containing supercritical water oxidation system

As the science and process applications of supercritical water (SCW) and supercritical water oxidation (SCWO) become more thoroughly understood, it is logical to envision the use of the SCWO process by diverse industries and public wastewater and sludge generators. This technology can be adapted to accomplish either pre or end-of-pipe wastewater treatment. There is a need to destroy both military and civilian hazardous waste, and urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. By capitalizing on the properties of water above its critical point, 374 °C and 22.4 MPa for pure water, this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While iron-based alloys and nickel-based alloys are generally considered important for service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking (SCC) and dealloying, are seen in chlorinated environments. This work assesses the corrosion characteristics of iron-based stainless steels exposed to high supercritical temperatures in a chlorinated military waste containing salts.     

Influence of impurities on stainless steels corrosion in wet process phosphoric acid

The corrosion behaviour of stainless steels in wet process phosphoric acid was studied by means of polarization curves and corrosion tests; in this context the influence of impurities that enhance the corrosivity of the acid, such as chlorides, fluorides and sulphates, was evaluated. Moreover the efficiency of some corrosion inhibitors, present as impurities in wet-process phosphoric acid, was evaluated by means of polarization curves and corrosion tests. Inhibitors of fluoride aggressiveness such as compounds of silicon, aluminum, magnesium were tested both separately and together. Trivalent iron as an oxidizing agent was also evaluated in order to minimize chloride-induced corrosion. Different grades of stainless steels such as AISI 316 L, Alloy 20, Alloy 28 and a duplex stainless steel 22Cr5Ni type, were taken into consideration.