Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

Chloridinduzierte Korrosion von Nichtrostenden Stählen in Schwimmhallen-Atmosphären Teil 1: Elektrolyt Magnesium-Chlorid (30%)

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres Part 1: electrolyte magnesium-chloride (30%) The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore U-bend specimens with salt spots were examined at 40°C and 35 and 70% rel. humidity respectively. A further test series was realised with round specimens under constant load in a stress cracking testing system at the same temperature and humidity conditions. The highly-nitrided steels 1.4529 and 1.4565 showed the best corrosion resistance. The steels 1.4401, 1.4462 and 1.4539 incured the highest corrosion attacks. Stress corrosion cracking (SCC) was determined at the steels 1.4401 and 1.4462 at 35% rel. humidity by metallographic structure micrography only. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack.     

Long-term corrosion behaviour of stainless reinforcing steel in mortar exposed to chloride environment

Long-term corrosion behaviour of six stainless reinforcing steels embedded in mortar and exposed to chloride media was monitored by electrochemical impedance spectroscopy at the open circuit potential during the period of 2 years. Corrosion behaviour of studied steels was divided into two phases characterized by different interfacial behaviour: (i) passive phase and (ii) pitting propagation phase. After 2 years, duplex steel 1.4362 showed very good corrosion performance similar to austenitic steel 1.4401. Steel types with low Ni content but with high N and Mn content, 1.4597 and 1.4162, showed lower corrosion resistance compared to austenitic steel 1.4301.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Kurzzeituntersuchungen zur Auswahl von rostbeständigen Cr-Stählen mit verbessertem Widerstand gegen Schwingungsrißkorrosion

Short-time investigations into the selection of stainless chromium steels with improved corrosion fatigue resistance Using the steel X 20 Cr 13 various short-time test methods have been tested for evaluating the corrosion fatigue susceptibility in concentrated air-saturated sodium chloride solution. In addition the steels X 15 CrNi17, X 35 CrMo 17, X 5 CrNiMoCuNb 14 5, X 4 CrNiCuNb 16 4, X 2 CrNiMoN 22 5 3, X 5 CrNiMoCu 25 5, X 8 CrNiMo 27 5 and X 1 CrNiMoNb 28 4 2 have been studied at room temperature of 80° C using crack propagation, constant straining and pitting corrosion methods. The two last mentioned steels are superiour under the test conditions.     

Herstellung und Eigenschaften stickstofflegierter,korrosionsbeständiger Stähle und Sonderstähle mit niedrigen Kohlenstoffgehalten

Production and properties of nitrogen alloyed, corrosion resistant steels and special steels with low carbon contents Alloying with nitrogen has favourable influence in particular on the mechanical properties of CrNiMo steels (X 2 CrNiMoN 17 12, materials No. 1.4406, X 2 CrNiMoN 17 13 5, materials No. 1.4439 und X 2 CrNiMoN 22 5, materials No. W.-Nr. 1.4462). This comes to bear when ambient temperature and low temperature strength and toughness are concerned. With respect to the corrosion behaviour the data concerning the effect of nitrogen are contradictory. It has become clear that nitrogen improves pitting corrosion resistance; this applies, however, only to pit initiation but not to pit growth. Stress corrosion cracking is not delayed by nitrogen but different results have been obtained with different media: while the duplex steel X 2 CrNiMoN 22 5 is attacked considerably faster than the corresponding nitrogen-free steel in 42% boiling magnesium chloride solution the time-to-failure of both steels are comparable in 30% boiling MgCl₂-solution. The nitrogen alloyed steels can be welded by all known welding procedures, provided fully austenitic welding rods are used.     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

Die Korrosionsbeständigkeit der nichtrostenden Stähle an der Atomosphäre – Auswertung von Versuchen bis zu 10jähriger Auslagerung–

Corrosion resistance of stainless steels at the atmosphere – Evaluation of the results of weathering tests up to 10 years' duration – The evaluation of results obtained in the course of weathering tests up to 10 years' duration has confirmed the conclusions drawn after one year's exposition. A CrNiMo steel (Werkstoff-Nr. 1.4401) can be used even in the most severe conditions; slight corrosions found in marine atmospheres can be attributed to unpolished surfaces. The 17% Cr steel is considerably attacked in industrial and marine atmospheres and should not be used for the external parts of buildings. 18/8 steels are perfectly resistant to urban atmosphere, with the exception of those portions which are not exposed to the cleaning effect of rain waters. In all the cases studied the corrosion resistance of the steels has been improved by electropolishing. Electrochemical studies have further revealed that the pitting corrosion susceptibility decreases in the same order as the atmospheric corrosion resistance.     

Untersuchung des SwRK-Verhaltens des hochreinen martensitaushärtenden Stahles X5CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X4CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 2: SwRK-Untersuchungen und Rißeinleitungsmechanismen

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel X5CrNiCuNb 17 4 PH in comparison to the soft martensitic stainless steel X4CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 2: Corrosion fatigue tests and crack initiation mechanisms The following report concerns the study of the corrosion fatigue behaviour of the soft martensitic steel X4CrNiMo 16 5 1 ESR and the precipitation hardened X5CrNiCuNb 17 4 PH in sodium solution in the temperature range between 20° and 150 °C and the determination of their general corrosion properties and the mechanism of crack propagation. Their corrosion fatigue limits were compared with each other. A comparision was also made between an electro-slag-remelted soft martensitic steel and a charge without an ESR aftertreatment. Microfractographical fracture and crack path investigation were carried out for interpretation of the experimental results. It was observed that in both super pure steels (soft martensitic and precipitation hardened) the oxidic inclusions are not responsible for the crack intiation, as it was found in the non ESR treated steels. In the 17-4 PH steel copper containing inclusions in the crack initiation areas were observed. In concentrated sodium solution pitting corrosion was found at both steels.     

Temporärer Korrosionsschutz von Spannstählen in unverpressten Hüllrohren

Temporary corrosion protection of prestressing steels in non‐injected ducts Prestressing steels may be subjected to corrosive conditions during the manufacturing process at building sites. Due to this a risk of hydrogen induced stress corrosion cracking of the steels may arise. Tests under practical conditions in a prestressed concrete beam were carried out where non‐injected ducts were treated with preheated scavenging air to prove this method being able to protect the prestressing steels against corrosion. The results yielded sufficient corrosion protection by this measure and therefore it may be an interesting alternative in comparison to corrosion protection by film forming agents which contain inhibitors.     

Untersuchung des SwRK-Verhaltens des martensitaushärtenden Stahls X 5 CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X 4 CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 1: Korrosionsuntersuchungen und SpRK-Untersuchungen zur Optimierung der Wärmebehandlung bezüglich der SpRK-Beständigkeit

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel (X 5 CrNiCuNb 17 4 PH) in comparison to the soft martensitic stainless steel X 4 CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 1: Corrosion investigations and stress corrosion tests to optimize the heat treatment according to the stress corrosion resistance The aim of this investigation was to improve the reliability of operation and the economy of corrosion fatigue stresses of structural elements, especially at elevated temperatures, by optimizing the material. This investigation is of great interest both to the industry and to related fields. It concerns the influence of a higher degree of purity achieved by the secondary metallurgical aftertreatment of soft martensitic steel X4CrNiMo 16 5 1 ESR (ESR-electroslag remelting) in comparison to the corrosion fatigue behaviour of the precipitation hardening steel X5CrNiCuNb 17 4 PH, which was specially heat-treated with respect to stress corrosion cracking resistance. The stress corrosion was investigated for all heat-treatments of the 17-4 PH in 22% NaCl(pH3) solution. The precipitation hardening steel was most resistant to stress corrosion in concentrated NaCl-solution after a three-stage heat-treatment. There was no improvement of corrosion fatigue resistance after metallurgical aftertreatment of soft martensitic steel compared to the untreated material. This is due to the instable passive behaviour of the material which led to crack initiation, especially during the 150°C experiments, at chloride-induced places of pitting. The investigation of the electrochemical corrosion behaviour of both materials showed that the pH-value hardly influences corrosion restistance. An increase of the salt content leads to higher pitting induction. At temperatures of 80°C in a saturated NaCl-solution the material showed no corrosion resistance. In potentiokinetic investigations, a direct transition from the active area to the pitting potential was observed. In accordance with both the corrosion fatigue and the stress corrosion cracking investigations, it was found that pitting at the martensite precipitator starts primarily around Cu-containing or oxidic inclusions.     

30 Jahre anodischer Korrosionsschutz

30 years anodic corrosion protection The technical use of the anodic corrosion protection is entering the fourth decade. A summary of the development in the past thirty years verifies the efficiency of this kind of protection. Damage to un- and low alloyed steel construction, specially on the aluminium producing industry, by intercrystalline stress corrossion cracking in alkaline solution are stopped by anodic corrossion protection. The anodic corrosion protection allows the application of low cost materials in a “passive” state, e.g. stainless steels (1.4301, 1.4401) in acid equipment, especially sulfuric acid units, under high corrosive conditions, and no damage will occur. The anodic corrosion protection enables the use of equipment at a very high level of safety and economy.     

Sonderstahle für den Apparatebau mit sehr guter Korrosionsbestandigkeit und erhöhter Festigkeit

Special steels with superior corrosion resistance and strength for chemical equipment manufacture Stainless steels are among the materials most predominantly used in chemical plant engineering. During the past few years, quite a number of special steels of this type have been developed to meet the eve1 increasing demands in this field. This paper deals with two of them. VEW A 963 is an austenitic CrNiMo steel containing about 6.3% molybdenum which shows superior resistance to the attack of media with a high chloride content, in other terms, excellent resistance to pitting and crevice corrosion. Stress corrosion cracking tests in NaCl solution with crust formation show steel VEW A %3 to be definitely superior to CrNiMo grades with up to 4.5% molybdenum. Corrosion resistance in acids, too, is very good. The steel possesses good weldability, and suitable filler metals are available in the form of coated electrodes and inert gas welding wire. Big scale production of steel VEW A 963 in the form of sheet and plate, bar, forgings and seamless tube is possible. VEW A 905 is an austenitic-ferritic CrNiMo steel with manganese and nitrogen additions which permit to obtain a minimum 0.2% proof stress of 590 Nlmm² in the solution annealed condition. The micro- structure shows more or less equal parts of austenite and ferrite which hardly change with rising temperature. This is of great importance for welding: there is no grain coarsening in the heat affected zone, nor increase of ferrite content. An electrode type of the same composition is available. VEW A 905 has good resistance to pitting corrosion and to the attack of a variety of acids. Particular emphasis should be laid on its excellent resistance to chloride induced stress corrosion cracking. VEW A 905. too, is available in the form of sheet and plate, bar, forgings and seamless tube produced on a large scale.     

Lateral and radial corrosion propagation behavior of 9–21% Cr and 18% Cr + 2.8% Mo stainless steel reinforcing materials in simulated concrete environments

The life of a concrete structure exposed to deicing compounds or seawater is often been limited by chloride induced corrosion of the steel reinforcement. A complete assessment of the potential benefits afforded by new candidate rebar alloys must address both the lateral and radial corrosion propagation behavior in comparison to conventional steel as well as other factors that might affect the risk of corrosion‐induced concrete cracking. The radial (depth) and lateral (length) corrosion propagation behavior of 18% Cr + 2.8% Mo (S31653) stainless steel, 21% Cr (S32101) duplex stainless steel, and 9% Cr steel compared to plain ASTM A615 carbon steel were characterized in saturated Ca(OH)₂ solution. Radial pit growth was found to be Ohmically controlled for all materials but repassivation occurred more readily at high applied potentials for 18% Cr + 2.8% Mo and 21% Cr stainless steels. Conversely, pit growth on plain steel propagated at all applied anodic potentials and did not repassivate until deactivation by cathodic polarization. Stainless steel also showed the highest resistance to lateral corrosion propagation from an active site during microelectrode array testing. 21% Cr duplex stainless and 9% Cr steel showed similar radial propagation behavior and corrosion morphology, which was intermediate to that of plain steel and S31653 stainless steel. Based on an existing concrete cracking model, it is expected that 9–21% Cr and 18% Cr + 2.8% Mo corrosion resistant rebar materials would require a greater depth of corrosion attack than carbon steel before damaging concrete via corrosion product formation.     

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.     

Stainless steels: Passive film composition,pitting potentials,and critical chloride content in concrete

Stainless steel reinforcing bars show excellent corrosion resistance in concrete structures exposed to harsh environments. Only a little information on the surface chemistry of these materials in alkaline media is available. This study reports the X-ray photoelectron spectroscopy surface analytical results (thickness, composition of the passive film and of the interface beneath the film) obtained on black steel, FeCr alloys, DIN 1.4301, DIN 1.4462 and the nickel-free DIN 1.4456 after exposure to alkaline solutions simulating concrete. The pitting potentials of the steels could be related to the Cr(III)oxy-hydroxide and Mo(VI) content in the passive film. C_crit, the critical chloride content for corrosion initiation in concrete, necessary for life-time predictions, can be determined only with time-consuming tests, especially for high-alloyed stainless steel. This study reports a correlation between C_crit in concrete (made with CEM II-A/LL and CEM I) and the pitting potential for carbon steel, Fe12%Cr alloy, DIN 1.4301, and DIN 1.4571 stainless steels. This could allow, for the first time, a quantitative estimation of C_crit for stainless steel in concrete based on short-term solution tests.     

Untersuchungen über den Ablauf der Spannungsrißkorrosion an austenitischen Chrom-Nickel-Stählen in Magnesiumchloridlösung Mitteilung aus dem Forschungsinstitut der Mannesmann AG

Investigation into the cinetics of stress corrosion cracking of austenitic chromium-nickel steels in magnesium chloride solution Experimental investigation with the steels (German designation) W.-Nr. 4301 (X5CrNi18 9), 4401 (X5CrNiMo18 10), 4449 (X5CrNiMo17 13), 4541 (X10CrNiTi18 9), 4550 (X10CrNiNb18 9) and 4580 (X10CrNiMoNb18 10) in 42% MgCl₂ solution at 144 and 130°C. The known distribution of stress corrosion cracking between an incubation and a crack propagation period has been confirmed (the first taking about 10% of the whole specimen life). The cracking rate is proportional to the potential at the base of the crack which, in turn, depends from the surface potential. Nb and Ti have no bearing on the cracking rate. Addition of Mo displaces to more noble potentials the potential limit below which there is no stress corrosion cracking and reduces cracking rates when differences between the potential limit and a given potential are equal. The influence of temperature is restricted to the crack propagation period. A co-action of increasing Ni contents in connection with the Mo addition cannot be excluded.     

Untersuchungen über Korrosionsvorgänge an Stahl nach einmaliger Chlorid-Kontamination

Investigations into the corrosion processes on steel after one-time's chloride contamination Investigation of chloride adsorption and desorption on mild steel St 37 (1.0120) and austenitic stainless steel X 5 CrNi 189 (1.4301) showed chloride adsorption layer, determined radiochemically, depends upon the material, the state of metal surface layer, and its pretreatment. Ground surface of St 37 retains comparatively less chloride ions on washing than an unpretreated specimen. On the other hand the chloride could be removed from the austenitic steel by rinsing twice in unagitated washing medium which lowered the chloride content to below the detection level of 0,02 m?g Cl^?/cm². Corrosion behaviour of specimens of 1.4301 in autoclave showed no difference with regards to corrosion in the test cycles of 300 and 400 h at 150 and 200° C whether or not the specimens were contaminated by chloride. Mild steels whose state of surface is similar to St 37 should be guarded against chloride contamination to prevent delayed damage. In case of steel similar to 1.4301 with regards to their surface characteristics cleaning the surface is relatively easy. Long time tests to assess risks of delayed damage are in progress and will be communicated later.     

A comprehensive assessment of the performance of corrosion resistant alloys in hot acidic brines for application in oil and gas production

Four stainless steels and alloys (17-4 PH, X4CrNiMo 16-5-1, F6NM and UNS N09935) were evaluated in relation to their application in the oil and gas industry. These materials were tested in solutions exhibiting a range of chloride concentrations, pH values and temperatures of interest for the oil and gas producing environments. The pitting sensitivity was investigated by means of potentiodynamic polarisation measurements, based on the ASTM G61 standard, in conjunction with a morphological study performed by scanning electron and optical microscopy. The resistance to stress corrosion cracking (SCC) was evaluated in compliance with the ASTM G123 standard. Erosion–corrosion was assessed by exposing the materials under electrochemical control to a flux of erodent glass microspheres in a rotating disc electrode device. A ranking of the materials resistance was derived, based on appropriate parameters, devised to effectively and synthetically represent the complex sets of environments of interest for the relevant application. Our results showed, as expected, that UNS N09935 displays the best performance with respect to pitting resistance and susceptibility to SCC as well as a very good resistance to erosion–corrosion. Among the other investigated materials, 17-4 PH showed higher resistance to pitting, X4CrNiMo 16-5-1 and F6NM longer time to SCC failure while 17-4 PH and X4CrNiMo 16-5-1 exhibited superior ability to withstand erosion–corrosion damaging.     

Untersuchungen über den Einfluß des Gefügezustandes auf das Korrosionsverhalten hochlegierter,rost- und säurebeständiger Stähle in reiner sowie in chloridhaltiger Schwefelsäure

Corrosion Properties of High Alloyed Stainless Steels in Pure as well as in Chloride Containing Sulfuric Acid The corrosion behaviour of the high alloyed stainless steels material no. 1.4439 (X3CrNiMoN17135), 1.4539 (X2NiCrMoCu25205), 1.4503(X3NiCrMoCuTi2723) as well as the reference materials AlSI 316 L and alloy 825 was tested in diluted sulfuric acid (5, 10, 20 and 50%) at 50, 100 and 150°C. The test solutions additionally contained impurities as chlorides and cupric ions. On the material side the effect of various microstructures was checked as well: material as received (commercial production), solution annealed under laboratory conditions, cold deformed and for two selected steels electroslag remelted. Corrosion testing methods are: the immersion test will sheet coupons and the measurement of the weightloss; electrochemical testing, i.e. Current potential-and free corrosion potential-time-curves. No pitting corrosion is observed in the presence of chloride ions. In some cases the general corrosion rate is lowered if chloride ions are present. This beneficial effect of chloride ions, however, is observed only at low chloride concentrations (500 ppm). Annealing under laboratory conditions as well as electroslag remelting does not generally improve the corrosion resistance. A negative effect by cold deformation is only observed for standard stainless steel AlSI 316. Cupric ions added to the 20% sulfuric acid solution improve the corrosion resistance of all steels investigated to that extent, that they can be used in practice up to 100°C provided that the concentration of cupric ions in the solution is sufficiently high (2000 ppm). Electrochemical test results indicate that the positive effect of cupric ions is due to the shift of the free corrosion potential into the potential range of stable passivity. Copper alloyed stainless steels show the highest corrosion resistance.