Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Surface spreading of intergranular corrosion on stainless steels

The spreading of intergranular corrosion was investigated using micrometer scale simulations and experimental verifications on sensitized stainless steel [UNS S30400]. The degree of sensitization, presence of a pit, and applied potential all affected spreading. The inputs used in the simulation were obtained from Fe-XCr(X = 10, 12, 14, 16 wt.%)-Mo-Ni alloys representing various grain boundary Cr depletion levels. Corroding grain boundaries and pits triggered corrosion of nearby sensitized boundaries due to Ohmic potential drop. Large connected clusters of corroding grain boundaries formed at high fractions of Cr-depleted grain boundaries. The metallurgical, electrochemical and geometric conditions for this behavior could be forecasted.     

Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels

Intergranular corrosion (IGC) and precipitation of ferritic stainless steels (FSS) were investigated with change in Cr content from 11 wt.% to 17 wt.%. The increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC. The analysis on the intergranular precipitates revealed that Cr segregation around fine intergranular TiC in developed all FSS regardless of Cr content. This Cr segregation is proposed to explain the Cr depletion for the cause of IGC in Ti-stabilized Cr FSS.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Effects of inclusions on the precipitation of chi phases and intergranular corrosion resistance of hyper duplex stainless steel

During the initial stage of aging heat treatment at 850 °C, inclusions such as (Cr, Mn, Al) oxides and (Cr, Mn, Al, Fe) oxides of a hyper duplex stainless steel act as preferential precipitation sites for the chi phase like ferrite/austenite phase boundaries and ferrite/ferrite grain boundaries. The chi phase is precipitated around the inclusions due to the blocking and piling up the alloying elements such as Mo and W around the inclusions. The precipitation of Mo and W enriched chi phase around the inclusions decreases the intergranular corrosion resistance due to the formation of Mo and W depleted zones.     

A study of intergranular corrosion of austenitic stainless steel by electrochemical potentiodynamic reactivation, electron back-scattering diffraction and cellular automaton

The impact of solution and sensitization treatments on the intergranular corrosion (IGC) of austenitic stainless steel (316) was studied by electrochemical potentiodynamic reactivation (EPR) test, and the results showed the degree of sensitization (DOS) decreased as solution treatment temperature and time went up, but it increased as sensitization temperature prolonged. Factors that affected IGC were investigated by field emission scanning electron microscope (FE-SEM) and electron back-scattering diffraction (EBSD). Furthermore, the precipitation evolution of Cr-rich carbides and the distribution of chromium concentration were simulated by cellular automaton (CA), clearly showing the effects of solution and sensitization treatments on IGC.     

Grain boundary structure and intergranular stress corrosion crack initiation in high temperature water of a thermally sensitised austenitic stainless steel,observed in situ

The development of an intergranular stress corrosion crack initiation site in thermally sensitised type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations. The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al., applying three-dimensional data for the grain boundary planes and grain orientations. The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility.     

Corrosion behaviour of intermetallic aluminide coatings on nitrogen-containing austenitic stainless steels

The present work reports the effect of aluminide layers on the aqueous corrosion behaviour of four different 316L stainless steels containing various nitrogen contents (0.015%, 0.1%, 0.2% and 0.56% N). Diffusion annealed aluminide layers are generated over the surface by heat treatment of the aluminium precoated alloys at 750 °C for 25 h in nitrogen atmosphere. X-ray diffraction patterns of the surface modified samples showed the presence of AlN, Al₁₃Fe₄ and FeAl₂ phases. Diffusion of aluminum into the alloy, and the formation of AlN by the reaction of aluminium with matrix nitrogen, was identified using secondary ion mass spectrometry (SIMS). The nitrogen peak in the diffused layer was found to increase with increasing nitrogen content of the base alloy. SEM observation of cross-sectionally mounted alloys showed the presence of spherical AlN phase in addition to iron aluminide intermetallic phases. The role of such a composite surface layer containing intermetallic aluminides and nitride on the corrosion resistance of austenitic stainless steels in 0.5 M NaCl and 0.5 M sulphuric acid is discussed in greater detail based on open circuit potential (OCP)–time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The aluminide layered alloy with 0.1% N content showed better corrosion performance. The presence of nitrogen was found to have a positive effect in enhancing the hardness of the composite layer. Role of matrix nitrogen on the microstructure and microchemical distribution at the surface, and its role on corrosion resistance in acidic and chloride media are discussed in detail.     

Intergranular corrosion of welded joints of austenitic stainless steels studied by using an electrochemical minicell

An intergranular corrosion study of welded joints of austenitic stainless steels (AISI 304 and 316L) has been addressed. A specific small-scale electrochemical cell (minicell) has been used. Four different weldment zones have been studied. The electrochemical methods applied were the electrochemical potentiokinetic reactivation test and electrochemical potentiokinetic reactivation double loop test. These techniques showed that the HAZ was the most critical zone to intergranular corrosion for both materials. The weld metal was susceptible to interdendritic corrosion and the fusion line showed a mixture of intergranular and interdendritic corrosion. The effect of pre- and post-welding heat treatments for AISI 316L was analyzed. The HAZ was again the most critical zone in every heat treatment condition. The results were correlated to the microstructural features of the materials.     

Pitting corrosion behaviour of austenitic stainless steels with Cu and Sn additions

The influence of Cu and Sn on the pitting corrosion resistance of AISI 304 and 316 stainless steels in chloride-containing media has been investigated. The corrosion behaviour was evaluated by cyclic polarization, potentiostatic CPT measurements and electrochemical impedance spectroscopy in 3.5 wt% NaCl. The corrosion resistance was also studied in FeCl₃ under Standard ASTM G-48. According to the results, Cu addition favours pit nucleation but inhibits its growth, whereas Sn exerts the opposite effect, favouring pit growth and inhibiting its nucleation. Studies by SEM, X-ray mapping and EDS analysis showed Cu-, Cl- and O-rich corrosion products that reduce the extent of corrosion damage.     

Pitting corrosion behaviour of austenitic stainless steels - combining effects of Mn and Mo additions

Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media. Corrosion behaviour was investigated using gravimetric tests in 6 wt.% FeCl₃, as well as potentiodynamic and potentiostatic polarization measurements in 3.5 wt.% NaCl. Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis. The beneficial effect of Mo additions was assigned to Mo⁶⁺ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl⁻ ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation. Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators.     

Martensitic transformation in an intergranular corrosion area of austenitic stainless steel during thermal cycling

An oxidation-assisted martensitic phase transformation was observed in an austenitic stainless steel after thermal cycling up to 970 °C in air in a solar thermal steam reformer. The intergranular corrosion areas were investigated by electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). The structural-and-chemical maps revealed that within intergranular corrosion areas this martensitic transformation primarily occurs in oxidation-induced chromium-depleted zones, rather than due to only sensitization. This displacive transformation may also play a significant role in the rate at which intergranular corrosion takes place.     

Pitting corrosion behaviour of PM austenitic stainless steels sintered in nitrogen-hydrogen atmosphere

PM 304L and 316L stainless steel have been compacted at 400, 600 and 800 MPa and sintered in vacuum and in nitrogen-hydrogen atmosphere. Postsintered heat treatments (annealing solution and ageing at 375, 675 and 875 °C) have been applied. Pitting corrosion resistance has been studied using anodic polarization measurements and the ferric chloride test. Anodic polarization curves reveal that densities and atmospheres are relevant on anodic behaviour. Pitting resistance is higher for PM 316L and for higher densities and vacuum as sintered atmosphere. Ageing heat treatments at medium and high temperatures are detrimental to passivity although susceptibility to pitting corrosion barely changes. But heat treatments at 375 °C even show certain improvement in pitting corrosion resistance. The results were correlated to the presence of precipitates and mainly to the lamellar constituent which appears in some samples sintered in nitrogen-hydrogen atmosphere. The role of nitrogen on the samples sintered under nitrogen-hydrogen atmosphere and its relation to the microstructural features was described.     

Application of the modified electrochemical potentiodynamic reactivation method to detect susceptibility to intergranular corrosion of a newly developed lean duplex stainless steel LDX2101

The present work aimed at defining optimal conditions using double loop electrochemical potentiokinetic reactivation (DL-EPR) method for evaluating intergranular corrosion (IGC) susceptibility of lean duplex stainless steel (LDX2101) aged at 700 °C between 3 min and 300 h. The results demonstrated that the modified DL-EPR measurement (solution of 33% H₂SO₄ + 0.1% HCl at 20 °C and scan rate of 2.5 mV/s) could successfully characterize the interactions between precipitation, chromium depletion and IGC of LDX2101 with high sensitivity and reproducibility. In addition, there was no indication of healing because the effect of formation of chromium-enriched precipitates was more dominative than that of redistribution of chromium in depleted zones.     

Electrochemical behaviour of duplex stainless steels in caustic environment

Recent studies have shown that duplex stainless steels can be susceptible to general corrosion and stress corrosion cracking in high pH caustic environments. This difference in the corrosion resistance can be attributed to changes in the electrochemical behaviour of steels. The present study has shown that the corrosion rates of duplex stainless steels increase with an increase in temperature and sulphide addition to caustic environments. Moreover, alloying Fe with Cr and Ni helps to raise the corrosion potential and lower critical current density of DSS in an alkaline environment whereas Mo can be detrimental to the corrosion resistance of the steel.     

Studies of the tensile and corrosion fatigue behaviour of austenitic stainless steels

Corrosion fatigue behaviour of four types of austenitic stainless steels were investigated in boiling 45% magnesium chloride solution at a stress ratio of 0.25 and a frequency of 0.1 Hz. Type 316LN stainless steel possessed the best resistance and type 304 stainless steel had the lowest resistance to corrosion fatigue. XPS studies on the fracture surface indicated that the presence of nitrogen as ion in the surface film of type 316LN stainless steel gave it the highest resistance to corrosion fatigue. Fractographic examination showed wholly transgranular cracking in all cases.     

The effect of large heat input on the microstructure and corrosion behaviour of simulated heat affected zone in 2205 duplex stainless steel

In the simulated heat affected zone of 2205 duplex stainless steels, effects of large welding heat inputs on the microstructure and corrosion behaviour were investigated. Reformed austenite content increased with the coarsening of grain boundary austenite (GBA) and the growth of intragranular austenite (IGA) and Widmanstatten austenite (WA), thus improving the low temperature toughness and affecting corrosion state. Reduction of chromium nitrides contributed to better resistance to pitting corrosion. Moreover, the pitting corrosion and intergranular corrosion were improved resulting from the formation of more GBA and WA. The specimen with a Δt_8/5 of 100 s presents better comprehensive performance.     

Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

The corrosion behaviour of austenitic and duplex stainless steels in artificial saliva with the addition of fluoride

The evolution of the passive films on 2205 duplex stainless steel (2205 DSS) and AISI 316L stainless steel in artificial saliva, and with the addition of fluoride, was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements. The extent of the passive range increased for the 2205 DSS compared to the AISI 316L in both solutions. The formation of the passive film was studied by EIS at the open-circuit potential (OCP). The passive layers were studied at the OCP by X-ray photoelectron spectroscopy (XPS). The passive films on both materials predominantly contained Cr-oxides, whereas the Fe species were markedly depleted.     

Effect of marine Pseudoalteromonas sp. on the microstructure and corrosion behaviour of 2205 duplex stainless steel

The effects of isolated marine Pseudoalteromonas sp. on the microstructure and corrosion behaviour of 2205 duplex stainless steel were investigated using electrochemical and surface analysis methods. Electrochemical studies demonstrated a negative shift in corrosion potential and an increase in corrosion current density in the presence of bacteria. EDS results showed a high concentration of chloride ions in the biofilm structure and a decrease in Cr content beneath the biofilm layer and near cracks. These results could lead to localised corrosion on metal surfaces. FESEM images illustrated the process of bacterial attachment on the metal surfaces at different exposure times.