Effect of carbon and silicon on the structure and corrosion resistance of 18cr-8ni cast steel in concentrated nitric acid solutions

The solutions of nitric acid can be divided into two concentration ranges: range I from 0 to 60–70% and range II from 80 to 100%. The 18Cr-8Ni cast steel with about 4% Si and up to 0.12% C exhibits very good corrosion resistance in concentrated nitric acid solutions (up to 98%); however, it is slightly inferior to a corresponding 18 Cr-13 Ni grade. The investigations were concentrated on the effect of the carbon and silicon contents on structure and corrosion resistance of an 18 Cr-8 Ni cast steel in cold nitric acid solutions. The following materials were used in these studies: a cast stainless steel of type 18Cr-8Ni with 0.48–6.75% Si and 0.021–0.220% C. A detailed analysis of the main result shows that

• 1. in cold HNO₃ solutions of concentrations above 80% the silicon content in a cast steel must not be lower than 2.5-3.0%;
• 2. the 18Cr-8Ni cast steel containing more than 4% Si and up to 0.12-0.13% carbon is practically not susceptible to corrosion in cold HNO₃ over the whole range of concentrations.

     

Einfluß von Verformung und spannungsinduzierten Ausscheidungen auf die Korrosionsbeständigkeit von siliciumlegiertem,nichtrostendem Stahl X2CrNiSi 18 15 in Salpetersäure

Effect of deformation and stress-induced precipitations on the corrosion resistance of silicon alloyed stainless steel X2CrNiSi 18 15 in nitric acid The corrosion resistance of silicon alloyed X2 CrNiSi 18 15 stainless steel with about 4 wt.-% Si in nitric acid depends on both deformation degree of the material and number and distribution of precipitations in the microstructure. After cold deformation and subsequent heat-treatment at 700°C (1292°F), the corrosion rate in 5 M HNO₃ is higher than that of non-deformed material. This result was established with specimens isolated from each other but was particularly pronounced with specimens being in galvanic contact. By deformation and subsequent annealing at 700°C, precipitations are formed at the grain boundaries the number of which increases with increasing degree of deformation. With higher degree of deformation, the precipitations are preferably arranged at slip-lines. They were identified as chromium carbide, M₂₃C₆, and silicide of the Cr₅Ni₃FeSi₂ type. It is shown by electrochemical measurements that these precipitations stimulate the cathodic partial reaction, i.e., the reduction of nitric acid. Hence, the corrosion resistance of the silicon alloyed stainless steel is impaired by deformation and stress-induced precipitations formed during annealing at 700°C.     

The influence of the ferritic and austenitic phases and of silicon on the Corrosion resistance of alloyed cast steels

The paper deals with the influence of the α/gamma; phase ratio and silicon on the corrosion resistance of low carbon alloyed cast steels in strongly oxidizing media. Electrochemical investigations in 65% HNO₃ + 5g Cr⁶⁺ ions/I showed that none of the four tested cast steels, with γ_R = 100%, γ_R = 75%, γ_R = 50% and γ_R = 0%, achieved a passive state, and that the corrosion process was under cathodic control. Under such conditions the calhodic activation overpotential is considerably greater than the anodic activation overpotential, and, as well as this, the Tafel coefficient for a cathodic reaction has higher values than those which are characteristic for an anodic reaction. The austenitic cast steel is the most resistant to corrosion, since for the corrosion process to take place the greatest activation energy is needed. The results of investigations of corrosion resistance in 12M HNO₃, at 115°C, for 240 hours, confirmed the results of the electrochemical measurements and the predominant role of the austenitic cast steel with γ_R = 100%. By means of AES analysis it was found that the good resistance to corrosion of such a cast steel is based on the building up of a thin mechanically-protecting barrier, which mainly consists of silicon.     

Korrosionsprobleme in chlorhaltigen Medien: Lösungsmöglichkeiten durch einige nichtrostende Spezialstähle

Corrosion problems in chloride containing media: possible solution by some stainless special steels The increasing water pollution forces the chemical industry to use water with increasing chloride content for cooling and other purposes. This trend brings about increasing corrosion danger, in particular pitting, stress corrosion cracking and corrosion fatigue as well as crevice corrosion. The present paper deals with some steels characterized by resistance to these specific corrosion phenomena. A steel containing (%) 21 Cr., 7.5 Ni, 2.5 Mo, 1.5 Cu, to 2 Mn, to 1 Si and 0.06 C is particularly resistant to stress corrosion cracking. It contains 30 to 50% ferrite in an austenitic matrix. Even in Mg chloride solutions it may be kept under a load of 7 kg/mm² without stress corrosion occurring (with a steel of the 18 10 CrNiMo type the admissible load is only 2 kg/mm²). A steel containing (%) 25 Ni, 21 Cr, 4.5 Mo, 1.5 Cu, to 1 Si, to 2 Mn, and 0.02 C has a broad passivity range and is resistant to general corrosion in acid reducing media and phosphoric acid of all concentrations. A ferritic steel containing (%) 26 Cr. 1 Mo and minor additions of C, Mn, Si, Cu, Ni and nitrogen is resistant to stress corrosion cracking in neutral chloride solutions and general corrosion in oxidizing and neutral media, even against hydrogen sulfid and organic acids; it is beyond that lergely resistant to pitting in chloride solutions.     

Kurzzeitprüfmethoden zur Untersuchung des Einflusses von Wärmebehandlung und chemischer Zusammensetzung auf die Korrosionsbeständigkeit martensitischer Chrom-Nickel-(Molybdän)-Stähle mit tiefem Kohlenstoff-Gehalt

Short-duration test for evaluating the influence of heat-treatment and chemical composition on the corrosion resistance of low carbon martensitic chromium nickel (molybdenum) steels The following procedures were examined and compared for their suitability as rapid corrosion tests in comparative investigations on the influence of heat treatment and other production parameters on martensitic chromium-nickel-(molybdenum) steels of low carbon content:

• –Measurement of the mass loss in boiling 20% acetic acid, and in some cases 5% nitric or 1% hydrochloric acid.
• –Immersion in sulphuric acid/copper sulphate solution (modified Strauss test).
• –Current density/voltage curves in 1 N sulphuric acid.
• –Measurement of pitting potential und activation pH in 3% sodium chloride solution (on limited scale).

The test materials employed contained 0.032–0.082% carbon, 11.6–16.4% chromium, 3.8–5.9% nickel and 0.31–1.6% molybdenum. The most suitable testing procedures were selected according to the steel type. Up to a mass loss rate of ≥ 0.05 mg · cm^?2 · h^?1 (materials with resistance inferior to steel type 13-6-1) the acetic acid test provides an excellent basis for comparison. Provided the optimal testing temperature is adopted, current density/voltage curves and immersion in sulphuric acid/copper sulphate solution yield usable results with all steel types investigated. The value of the information yielded by the test procedures is generally enhanced by metallographic characterization of the corroded surfaces; in particular the localization of chromium-depleted structure zones is made possible by this.     

Vergleich des elektrochemischen Verhaltens der Stähle CrMnN 18 12 und CrNi 18 10 in Salpetersäure

Comparison of the electrochemical behaviour of the steels CrMnN 18 12 and CrNi 18 10 in nitric acid The electrochemical behaviour of the steels CrMnN 1812 and CrNi 18 10 in nitric acid has been studied by current-potential curves atomic absorption analysis of the solution and impedance spectroscopy. It has been found that in nitric acid solutions the corrosion resistance of the steel CrMnN 18 12 is comparable with that of the steel CrNi 18 10. Passivation of the steels starts at the corrosion potential. The two-step passivation range is probable attributable to a phase transformation in the oxidelayers formed on the steel surface. The analysis of the impedance curves allowed the establishment of an equivalent circuit for the passivation process. The double layer capacity, the polarization resistance, the Ohmic resistance and the capacity of the passive layer have been calculated.     

Der Einfluß von Stickstoff auf die korrosionschemischen Eigenschaften lösungsgeglühter und angelassener austenitischer 18/10 Chrom-Nickel- und 18/12 Chrom-Nickel-Molybdän-Stähle - II. Interkristalline Korrosion in Kupfersulfat-Schwefelsäure-Lösungen und in siedender 65% iger Salpetersäure

Influence of nitrogen on the corrosion behaviour of solution treated and annealed austenitic 18/10 chromium-nickel and 18/12 chromium-nickel-molybdenum steels. - II. Intercrystalline corrosion in copper sulfate sulfuric acid solution and in boiling 65% nitric acid Heat-treated steels containing 18.5 Cr, 11 Ni or 17.2 Cr, 13 Ni 2.6 Mo were investigated in the Strauß-Test (also in more rigid version) and the Huey-Test. The parameter studied is the precipitation behaviour and its influence on susceptibility to intercrystalline corrosion. The two methods of investigation differ with respect to the information obtainable; the influence of nitrogen e. g. can be recognized in the Huey-Test only. Nitrogen shifts the lower borderline of the grain disintegration field toward higher annealing temperatures and longer annealing times; the Z-phase appearing in the nitrogen and molybdenum containing steels influences the upper borderline and produces a significant increase of corrosion rate in nitric acid. The results are interpreted in terms of the chromium depletion theory.     

Influence of carbon content on wear resistance and wear mechanism of Mn13Cr2 and Mn18Cr2 cast steels

By means of impact abrasion tests, micro-hardness tests, and worn surface morphology observation via SEM, a comparison research based upon different impact abrasive wear conditions was conducted in this research to study the influence of different carbon contents(1.25 wt.%, 1.35 wt.%, and 1.45 wt.%) on the wear resistance and wear mechanism of water-quenched Mn13Cr2 and Mn18Cr2 cast steels. The research results show that the wear resistance of the Mn18Cr2 cast steel is superior to that of the Mn13Cr2 cast steel under the condition of the same carbon content and different impact abrasive wear conditions because the Mn18Cr2 cast steel possesses higher worn work hardening capacity as well as a more desirable combination of high hardness and impact toughness than that of the Mn13Cr2 cast steel. When a 4.5 J impact abrasive load is applied, the wear mechanism of both steels is that plastic deformation fatigue spalling and micro-cutting coexist, and the former dominates. When the carbon content is increased, the worn work hardening effect becomes increasingly dramatic, while the wear resistance of both steels decreases, which implies that an increase in impact toughness is beneficial to improving the wear resistance under severe impact abrasive wear conditions. Under the condition of a 1.0 J impact abrasive load, the wear mechanism of both steels is that plastic deformation fatigue spalling and micro-cutting coexist, and the latter plays a leading role. The worn work hardening effect and wear resistance intensify when the carbon content is increased, which implies that a higher hardness can be conducive to better wear resistance under low impact abrasive condition.     

Oberflächenbehandlung als Korrosionsschutzmaßnahme von nichtrostenden Stählen

Surface treatment as corrosion protection measure of stainless steels The pickling behaviour of several stainless austenitic steels and of one steel with ferritic/austenitic grain structure were investigated in pickling solutions of different compositions based on hydrofluoric acid. Because of uncertainties in practical applications, the influence of temperature, time and acid content on the mass loss is of high interest. In another series of experiments, aqueous solutions of citric acid were tested for their suitability as pickling chemicals for the materials X 6 CrNiTi 18 10 (AISI 321) and X 6 CrNiMoTi 17 12 2 (AISI 316 Ti). Finally, the pickling procedures based on nitric acid/hydrofluoric acid mixtures were compared with mechanical cleansing methods and with pickling procedures based on aqueous citric acid solutions as well, to elucidate their influence on the corrosion resistance of the treated materials. The valuation followed a pitting corrosion test in sodium chloride solutions of different concentrations after Herbsleb and Schwenk. Pickling with hydrofluoric acid solutions is superior to other cleansing procedures, if corrosive environments are present. The ecologically beneficial citric acid solutions are only able to remove the annealing colours from stainless steels.     

Korrosion nichtrostender austenitischer Stähle in (kondensierender) chloridhaltiger Salpetersäure

Corrosion of austenitic stainless steels in (condensing) nitric acid containing chlorides The corrosion parameters involved in the behaviour of austenitic stainless steels and nickelbase alloys in chloride containing nitric acid are being stated. Investigation of the corrosion resistance of the austenitic stainless steel 1.4306, ESR grade for application in nitric acid, in nitric acid condensates containing small amounts of chlorides. The condensates were formed from boiling nitric acids of molarities 6 to 10 containing 15 to 45 mg chlorides/1. The calculated corrosion rates of < 1 μm/y can be considered rather small. Especially in cases of insufficient wetting of the heat exchanger surfaces, however, local roughening and pitting corrosion is to be expected, mostly under rust-colored, chloride-containing deposits. The intensity of pitting corrosion increases with increasing chloride contents of the nitric acid solutions that evaporate. The sealing surfaces consisting of steel 1.4306 and PTFE did not exhibit any crevice corrosion. Nitric acid grade ESR-1.4306 definitely does not suffice requirements as described in this paper. It is suggested to use stainless steels which exhibit an improved resistance against chlorides and, at the same time, a high resistance against nitric acid attack. The following steel grades may be considered: X 1 CrNi 25 21 (Mat. No. 1.4335), X 2 CrNiMoN 25 22 2 (Mat. No. 1.4466) and/or X 1 NiCrMoCuN 31 27 4 (Mat. No. 1.4563).     

高碳硅锰铸钢中夹杂物的形成与控制

对高碳硅锰铸钢中夹杂物的主要类型进行了详细研究,发现高碳硅锰铸钢中除了常见的硫化物夹杂外,还存在含硅夹杂物、含铝夹杂物、含钙夹杂物和稀土夹杂物等。采用钢液高温出炉、吹氩处理以及吹氩后静置处理等工艺措施,可以明显减少高碳硅锰铸钢中的夹杂物,夹杂物含量可以控制在0.1%以下。高碳硅锰铸钢经吹氩去夹杂物处理后,韧性明显改善,尤其是冲击韧性提高21.2%,达到16.6J/cm2。     

Über das Verhalten einiger CrNiMo-Stähle beim Einsatz im chemischen Betrieb

The behaviour of some CrNiMo steels in use at chemical plants A report is given about the behaviour of some highly alloyed CrNiMo steels in use in inorganic-chemical plants. The observations are supported and supplemented by results of potentiostatic tests. In the presence of mixed acids, the corrosion resistance of the steels greatly depends, e.g., on the SO: Cl^? ion ratio. In the presence of Cl^? ions and at higher temperatures, exceeding about 70°C, the resistance is largely influenced by the specifically orientated analytical and structural pattern of the steels. Attention is drawn to the detrimental influence, especially in cast metals and welds, of the concomitant element, silicon, which — if encountered in increasing quantitities — favours the segregation of several corrosion-promoting phases. Examinations of case of damage in practical operation, supported by potentiostatic tests with CrNiMo steels, with and without copper contents, have shown that the presence of copper is apt to reduce the corrosion resistance in media containing hydrochloric acid or chlorine ions. On the other hand, a copper content may be beneficial in sulphate solutions free from chlorine ions.     

Korrosion nichtrostender Stähle und Nickelbasislegierungen in Salpetersäure-Flußsäure-Gemischen

Corrosion of stainless steels and nickel-base alloys in solutions of nitric acid and hydrofluoric acid Reactions involving nitric acid may always result in the contamination of this acid with fluorides. In highly concentrated nitric acid, the presence of small amounts of HF will substantially reduce the corrosion of metallic materials. Mixtures consisting of hydrofluoric acid and hypo-azeotropic nitric acid on the other hand will strongly attack: the metal loss will markedly increase with increasing HNO₃ and HF concentrations as well as with rising temperatures. The investigation covered 12 stainless steel grades and nickel-base alloys. With constant HNO₃ content, corrosion rates will rise linearly when increasing the HF concentration. With constant HF concentration (0.25 M), corrosion rates will increase rapidly with increasing nitric acid concentration (from 0.3 M to 14.8 M). This can best be described by superimposing a linear function and a hyperbolic function that is reflecting the change in the HNO₃ content. Alloys containing as much chromium as possible (up to 46 wt.%) will exhibit the best corrosion resistance. Alloy NiCr30FeMo (Hastelloy alloy G-30) proved to be well suitable in this investigation.     

Die Lochkorrosionsanfälligkeit des Vergütungsstahles X20Cr13 in Natriumchloridlösungen

Pitting corrosion susceptibility of the heat treatable steel X20Cr13 in sodium chloride solutions For blades in steam trubines it is common to use the heat-treatable steels with 13% Cr (X20Cr13). Cracking of blades in the region of the turbine, where the steam starts to get wet, have often been referred to be caused by corrosion fatigue. The fatigue cracks sometimes start at inclusions in the material, but more often at pits caused by pitting corrosion. Pitting is brought about by chloride containing blade deposits. To examine the pitting behaviour of the steel in NaCl-solutions we measured the mixed-potential, the potentiostatic piting potential, the mass-loss and the pits per area as depending on temperature (20–80°C), pH (5–7–9), oxygen content (20 μg/kg up to saturation with oxygen) and chloride content (8×10^?5 mole/l up to saturation with NaCl 5,4 mole/l). The results show that steel X20Cr13 suffers pitting corrosion because of it's low Cr content in all technical possible NaCl solutions down to a Cl content of 10^?5 mole/l at the mixed potential.     

Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.     

Der Einfluß von Stickstoff auf die korrosionschemischen Eigenschaften lösungsgeglühter und angelassener austenitischer 18/10 Chrom-Nickel- und 18/10 Chrom-Nickel-Molybdän-Stähle. - I. Ausscheidungsverhalten der Stähle

Influence of nitrogen on the corrosion behaviour of solution treated and annealed austenitic 18/10 chromium-nickel and 18/12 chromium-nickel-molybdenum steels. - I. Precipitation behaviour of the steels Investigations into the corrosion behaviour of CrNi steels with (weight-%) 0.019–0.029 C, 18.5 Cr, 11 Ni, 0.022–0.138 N and 0.020 to 0.031 Cr, 17.2 Cr, 13 Ni, 2.6 Mo, 0.028 to 0.130 N by the Strauß-Test, the Strauß-Test under rigid condition and the Huey-Test. The first part of this paper is devoted in particular to the analysis of structural constituents and in particular to precipitations. In the molybdenum-free steel heat treatment results in precipitation of M₂₃C₆ and Cr₂N, the precipitation of the first being independent from nitrogen content. In case of the molybdenum-bearing steels the precipitation of M₂₃C₆ is independent from nitrogen content too but is shifted toward higher temperatures. The precipitation of Cr₂N is slowed down by Mo. The precipitation of other phases (chi-, sigma- and Z-phases) is affected differently by nitrogen.     

Untersuchungen zum kritischen korrosionsauslösenden Chloridgehalt von Stahlfasern in künstlicher Betonporenlösung

Investigations into the critical corrosion‐inducing chloride content of steel fibres in artificial concrete pore solution It is well known, that reinforcement steel in concrete is normally protected against corrosion due to the high pH‐value of the pore solution of the concrete. This alkalinity leads to a passive layer on the steel surface, which prevents further corrosion. The passive layer can be destroyed by chloride ions diffusing into the concrete. The concentration of chloride in the concrete which leads to a destruction of the passive layer and therefore to corrosion of the steel is defined as the critical chloride content. Investigations in artificial concrete pore solutions show that the critical chloride content of black steel is strongly dependent on the pH‐value of the solution: the higher the concentration of the OH⁻‐ions the higher the critical chloride content. For steel fibres earlier investigations have shown, that steel fibres do not corrode in concrete even at high chloride contents. Therefore it could be assumed, that the critical corrosion‐inducing chloride content of steel fibres in concrete is distinctly higher than of conventional reinforcing steel. To verify this assumption the corrosion‐inducing chloride content of steel fibres is investigated in artificial chloride‐containing concrete pore solutions at different pH‐values. 5 different types of steel fibres, 1 lashing wire and as reference 1 reinforcing steel are investigated at 3 different pH‐value ranges. The concentration of chloride within the pore solution is gradually increased in time steps of 12 h. The beginning of corrosion is determined by current as well as potential measurements. Furthermore additional investigations are carried out with intermediate products of the fibre production (steel wires with different diameters) to investigate if the critical chloride content of the wires is increasing gradually with decreasing diameter. The investigations show, that steel fibres in artificial chloride‐containing pore solutions indicate an distinctly increased resistance against chloride‐inducing corrosion compared with conventional reinforcing steel for high pH‐values. With decreasing diameter of wires the critical chloride content increases gradually.     

SECM Study of Effect of Chromium Content on the Localized Corrosion Behavior of Low-Alloy Steels in Chloride Environment

This paper investigates the effect of chromium (Cr) content (0, 1, 3 and 5% Cr) in epoxy-coated alloy steel against corrosion using in situ electrochemical techniques such as EIS and SECM in a 3% NaCl solution. The EIS results revealed that the epoxy-coated Cr steel exhibited higher impedance values than carbon steel, which is attributed to the greater resistance of Cr steel toward corrosion. Based on the cyclic voltammogram results, the tip potentials were set at ?0.7, 0.04 and 0.60 V for determining the concentration of dissolved oxygen at cathodic region, and oxidation of Cr²⁺ and Fe²⁺ at anodic region, respectively. The SECM measurements showed that, the tip current in the anodic region has decreased with increase in Cr content of the sample, which indicates that the oxidation of Fe²⁺ and Cr²⁺ decreases (corrosion is reduced) with the increase in Cr content of the steel. Besides, 5% Cr steel can maintain the highest corrosion resistance, and 1 and 3% Cr steels have higher corrosion resistance than the 0% Cr steel. This higher corrosion resistance of Cr steel samples could be due to the formation of Cr-rich hydro-oxide layers [Cr(OH)₃ as a corrosion product] on the surface of the samples. Thus, the epoxy-coated Cr alloy steel has greater corrosion resistance in a chloride-containing environment than the carbon steel. Hence, epoxy-coated Cr alloy steel can be successfully used as a construction material in structures.     

Corrosion Behaviour of Manganese-Containing. Stainless steels. I. Galvanostatic measurements in H2SO4 solutions

The results of galvanostatic polarization experiments on four 17.3% Cr / 5.3% Ni stainless steels containing increasing amounts of Mn (upto 13.9%) are compared with those of an 18/8 stainless steel. All alloyes exhibit an initial step for the dissolution of Fe. A second arrest is recorded in dilute acid solutions and/or when applying high currents, and is related to the oxidation of the Cr. With an alloy containing traces of N₂, a third arrest is observed in dilute acid solutions; it is attributed to the oxidation of Cr₂N. The incorporation of Mn in CrNi steels is detrimental to their anti-corrosion characteristics. The content of this metal should be kept as low as feasible.     

Widerstand der Schweißverbindungen kohlenstoffarmer und mit Molybdän legierter korrosionsbeständiger Stähle gegen interkristalline und Messerlinienkorrosion

Resistance to intercrystalline and knifeline corrosion of welds in low carbon stainless steels containing molybdenum Welds in steels containing (%) 0,019–0,080 C, 1,00–1,56 Mn, 0,03–0,92 Si, 0–0,026 P, 0–0,018 S, 10–13,60 Ni, 16–20,50 Cr, 0–3 Mo, 0–0,057 Ti, 0–0,87 Nb, prepared by electroslag and automatic submerged arc welding are as a general rule not susceptible to intercrystalline and knifeline corrosion; a certain susceptibility encountered with the Mo containing types can be largely eliminated by sensibilizing at 650 °C. Differences in the corrosion behaviour between base metal and weld seam could not be encountered; in some cases the weld metal turned out to be even more resistant. The test solutions used were: sulfuric acid + Cu sulfate + Cu, boiling, 48 hrs, and 65% nitric acid, boiling, 3 · 48 hrs.