The corrosion control of ductile cast irons becomes a technological challenge when supplying castings to customers due to the high reactivity of this alloy in contact with air. An interesting alternative to the protective systems such as coatings or corrosion inhibitors included in packaging processes is the chemical modification of the cast alloys by means of alloying elements addition which are able to improve the corrosion resistance of ductile cast irons. Ni, Cr and Al added to the cast alloys significantly affect their structure and properties, among them their corrosion response, when exposed to air. It has been observed that Ni and Al improve the corrosion behaviour while Cr additionally promoted pearlite and carbides formation. The results from the corrosion tests performed on ductile cast iron alloys which contain these three elements are discussed in the present work.
El control de la corrosión de las fundiciones esferoidales supone un reto tecnológico a la hora de suministrar piezas a la industria por su alta reactividad en contacto con la atmósfera. Una alternativa interesante a los sistemas de protección que emplean inhibidores de corrosión vía aplicación directa o en embalaje es la modificación de la composición de la aleación con elementos de aleación que mejoren el comportamiento frente a la corrosión de las fundiciones esferoidales. Elementos como el Ni, Cr y Al en la composición de la fundición, aportan cambios significativos en la microestructura y en las propiedades de estos materiales, como su mejora en la resistencia a la corrosión atmosférica. Se ha observado que el Ni y el Al mejoran dicha respuesta a la corrosión, mientras que el efecto del Cr se manifiesta, además, en la estabilización de la perlita y la formación de carburos en la matriz, no observada en las aleaciones que contienen sólo los otros dos elementos. En el presente estudio se describen y discuten los resultados encontrados con fundiciones esferoidales modificadas con estos elementos.
Low-alloyed steels and cast irons are used to manufacture castings which are part of components used in different aggressive conditions (
When iron-based alloys are exposed to the atmosphere, the environmental parameters such as level of pollutants, mainly chlorides and SO2, temperature and relative humidity will play a crucial role on its corrosion behaviour. Because of its exposition, oxides, hydrated oxides, hydroxides together with other compounds that can be formed depending on the ionic pollutants present in the environment, are formed on the surface (
Metallic matrix in cast iron alloys are similar to steel alloy though graphite particles are also present in the former case. The shape, size and distribution of these particles are mainly affected by the composition of the alloy, the solidification path (stable versus metastable) and the cooling rates (
The present work deals with the evaluation against atmospheric and electrochemical corrosion of different ductile iron alloys with and without Ni, Cr and Al additions. Although it has long been known that high Cr concentrations increases drastically the corrosion resistance of cast irons (
The atmospheric corrosion test was carried out using plate castings which were manufactured with the 9 compositions included in
Alloy | C | Si | Mn | P | S | Cr | Ni | Mg | Al | Ti | Cu |
---|---|---|---|---|---|---|---|---|---|---|---|
C1 | 3.80 | 1.97 | 0.09 | 0.053 | 0.010 | 0.03 | 0.04 | 0.050 | <0.010 | 0.022 | 0.02 |
C2 | 3.73 | 1.94 | 0.12 | 0.045 | 0.009 | 0.04 | 0.50 | 0.050 | <0.010 | 0.025 | 0.02 |
C3 | 3.85 | 2.07 | 0.12 | 0.038 | 0.009 | 0.04 | 0.99 | 0.065 | <0.010 | 0.026 | 0.03 |
C4 | 3.75 | 1.99 | 0.13 | 0.040 | 0.013 | 0.50 | 1.02 | 0.058 | <0.010 | 0.026 | 0.02 |
C5 | 3.74 | 1.97 | 0.14 | 0.035 | 0.012 | 0.92 | 0.99 | 0.051 | <0.010 | 0.026 | 0.02 |
C6 | 3.79 | 1.94 | 0.07 | 0.063 | 0.012 | 0.03 | 0.03 | 0.050 | 0.015 | 0.017 | 0.03 |
C7 | 3.78 | 1.89 | 0.10 | 0.058 | 0.012 | 0.04 | 0.04 | 0.049 | 0.049 | 0.022 | 0.03 |
C8 | 3.65 | 1.92 | 0.11 | 0.050 | 0.011 | 0.05 | 0.04 | 0.049 | 0.057 | 0.024 | 0.04 |
C9 | 3.80 | 1.94 | 0.13 | 0.050 | 0.010 | 0.05 | 0.04 | 0.046 | 0.150 | 0.027 | 0.03 |
For each casting alloy, the batch melt was prepared in a 250 Hz medium frequency induction furnace (100 kW) with 120 kg as maximum capacity. In all cases, the metallic charges were prepared with 55% pig iron and 45% ferritic returns (these last coming from heavy-section castings produced for the eolic industry). Once accomplished the melting processes, the carbon and silicon contents of the obtained base alloys were adjusted adding high purity graphite from electrodes (wt.%, C = 98.8) and a FeSi alloy (wt.%, Si = 74.6; Ca = 0.3; Al = 0.7 and Fe = 24.4) respectively.
The different additions of Ni, Cr and Al into the base melts were performed by using high purity nickel briquettes (wt.%, Ni >99.9), a FeCr alloy (wt.%, Cr = 62.7; C = 7.5 and Fe = 29.8) and small aluminium ingots (wt.%, Al = 98.3). After completing the required additions in each case, the temperature of the base melts was increased to 1490-1500 ºC and about 50 kg were transferred to a 70 kg maximum capacity ladle to carry out the Mg-treatments according to the “sandwich” method. For this purpose, 0.6 kg of a FeSiMg 511 ferroalloy (wt.%, Si = 44.7; Mg = 5.6; Ca = 1.2; Rare Earths = 0.7 and Fe = 47.8) and then 0.3 kg of steel scrap pieces were introduced in a reaction chamber located in the bottom of the ladle before transferring the 50 kg of the base melt batches to it. Once finished the Mg-treatments, the resulting melt batches were skimmed while remaining into the ladle and one chemically bonded mould (see
(a) drag and cope of the moulds and (b) bunch containing the two plates
The inoculation process was carried out setting a piece of an inoculant ingot (wt.%, Si = 70-76; Al = 3.1-4.3; Ca = 0.3-1.3 y Rare Earths = 0.4-0.5) in the pouring cup just before filling each mould. In all cases, the inoculant addition was the 0.20 wt.% of the total amount of melt poured into the mould.
After cooling the casting plates up to room temperature they were removed from the bunches and then shot-blasted. In a subsequent step, the 9 plates “A” underwent an annealing treatment (heating to 920 ºC, isothermal period at 920 ºC for 1 h, cooling up to 300 ºC at 1ºC/min and cooling up to room temperature in open air) so as to obtain fully ferritic microstructures. A piece was obtained from each cast bunch (in the gating runner of plates “A”) to prepare a metallographic sample and to identify the constituents of the as-cast matrices. Once the plates were heat-treated and then cooled to room temperature, another metallographic sample was obtained from the gating areas of plates “A” to confirm that all microstructures were fully ferritic. These samples were also used to determine nodularity (Nod) and nodule count (N) values. Final data from these parameters were obtained from three different inspection fields at 100x per sample. In each image the total number and the area of each graphite particle were determined with the software ImageJ. In these analyses, all graphite particles with an area lower than 25 µm2 were not considered. Once these data were available, the graphite particles were assigned to classes III, V and VI according to ISO-945-1 and considering their circularity and Feret ratio (
Class | Circularity | Feret ratio |
---|---|---|
III | 0.00-0.60 | 2.0-1000 |
V | 0.50-0.77 | 1.0-1.5 |
VI | 0.77-1.00 | 1.0-1.5 |
Nod and N values were determined with equations (
Carbon and sulphur contents were determined by a combustion technique (LECO CS 200) while silicon contents were obtained by gravimetric methods. For the rest of elements, contents were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES Perkin Elmer Optima 5300).
An atmospheric corrosion test rack (see
The location selected to perform the exposure test had an atmospheric corrosivity category C3 according to the standard UNE-EN-ISO 12944-2:2018 (industrial and urban atmospheres) with frequent rainy days. The total exposure period elapsed between September and March with 186 days.
Parameter | Average | Maximum value | Minimum value |
---|---|---|---|
Temperature (ºC) | 11.9 | 19.5 | 1.5 |
Relative moisture (%) | 81.60 | 95.19 | 45.50 |
Rainfall (l/m2) | 6.7 | 62.1 | 0.0 |
NO2 (µg/m3) | 29.4 | 70.5 | 11.7 |
NO (µg/m3) | 23.3 | 74.8 | 3.5 |
SO2 (µg/m3) | 4.53 | 8.33 | 2.09 |
After the exposure period, the corrosion attack of the plates was characterised by metallographic analyses. For this purpose, all plates “A” were cut at 15 mm from the edge of the shortest side. Metallographic samples were obtained from these areas and the oxide layer thickness (CE) was determined. Each of these values was calculated as the average value from ten different width measures along the total oxide layer present in each sample. The corrosion rate (VC in g/cm2) values were then calculated assuming that all plates underwent uniform corrosion process, and no loss of the corrosion products layer did occur.
The composition of the oxides formed on the surfaces was analysed by Scanning Electron Microscopy, SEM (Zeiss Ultra Plus) with an EDS detector and by X-ray diffraction on polycrystalline samples (Bruker AXS D8 Advance). The X-ray diffraction patterns were registered under a CuKα radiation in the 2θ range 10-70º at 40 kV, 30 mA with a scanning rate of 0.6 º/min.
Electrochemical measurements were performed by triplicate on a conventional three electrode cell. The working electrode was the studied alloy while the reference electrode is an Ag/AgCl electrode (3M KCl) and the counter electrode was a platinum wire. The area of the working electrode was 1.20 cm2. The electrode was a 0.6 M NaCl solution. All the tests were carried out at 23 ± 2 ºC. Before testing, the samples were polished down to 600 grit.
Corrosion behaviour has been evaluated by potentiodynamic polarisation using an Autolab PGSTAT 302N potentiostat. The evolution of the open circuit potential (OCP) was recorded for 1 hour. The potentiodynamic curves were conducted at a scan rate of 0.166 mV/s. Before starting the scan, the samples remained in the solution for 20 min to stabilize the OCP. After this period the potential scan was started in the anodic direction from a potential value of -0.25 V with respect to the OCP, up to 0.25 V. Each test was repeated three times for a given sample to ensure the repetitiveness of the results.
The microstructural features observed in the 9 plates “A” before and after the heat-treatment are shown in
Alloy | Nod (%) | N (mm-2) | Heat treatment | Ferrite (%) | Pearlite (%) | Carbides (%) |
---|---|---|---|---|---|---|
C1 (base alloy) | 93.8 | 442.1 | No | 70-65 | 30-35 | 0 |
Yes | 100 | 0 | 0 | |||
C2 (0.50% Ni) | 93.2 | 545.1 | No | 50-60 | 50-40 | 0 |
Yes | 100 | 0 | 0 | |||
C3 (0.99% Ni) | 97.0 | 346.6 | No | 40-55 | 60-45 | 0 |
Yes | 100 | 0 | 0 | |||
C4 (0.50% Cr y 1.02% Ni) | 95.6 | 500.2 | No | 20-25 | 78-73 | 2 |
Yes | 30-35 | 68-63 | 1-2 | |||
C5 (0.92% Cr y 0.99% Ni) | 95.6 | 369.0 | No | 0 | 93-90 | 7-10 |
Yes | Traces | 95-90 | 5-10 | |||
C6 (0.015% Al) | 93.4 | 369.4 | No | 70-65 | 30-35 | 0 |
Yes | 100 | 0 | 0 | |||
C7 (0.049% Al) | 98.0 | 450.0 | No | 70-75 | 30-25 | 0 |
Yes | 100 | 0 | 0 | |||
C8 (0.057% Al) | 94.7 | 445.2 | No | 80-85 | 20-25 | 0 |
Yes | 100 | 0 | 0 | |||
C9 (0.150% Al) | 91.4 | 353.6 | No | 90 | 10 | 0 |
Yes | 100 | 0 | 0 |
The effects of alloying with Ni or Al have shown to be the opposite according to the ferrite and pearlite contents in the as-cast plates C2 and C3 versus C6 to C9 (see
As it was mentioned before, the heat-treated plates show matrices only composed by ferrite (
High magnification images (c) and (d) show the presence of pearlite and carbides.
After one week exposed to the atmosphere, the surfaces of all plates already showed clear evidence of corrosion. Thus, non-homogeneous, thin and brown-orange oxides appeared on the surface which suggested the formation of iron oxides (
Alloy | CE (mm) | VC (g/m2) | ||
---|---|---|---|---|
Average value | σ | Average value | σ | |
C1 (reference composition) | 92 | 15.6 | 167.9 | 28.4 |
C2 (0.50% Ni) | 78 | 7.1 | 142.4 | 12.9 |
C3 (0.99% Ni) | 66 | 14.1 | 120.5 | 25.8 |
C4 (0.50% Cr y 1.02% Ni) | 48 | 5.7 | 87.6 | 10.3 |
C5 (0.92% Cr y 0.99% Ni) | 40 | 7.1 | 73.0 | 12.9 |
C6 (0.015% Al) | 71 | 8.5 | 129.6 | 15.5 |
C7 (0.049% Al) | 67 | 7.1 | 122.3 | 12.9 |
C8 (0.057% Al) | 57 | 8.5 | 104.0 | 15.5 |
C9 (0.150% Al) | 28 | 4.2 | 51.1 | 7.7 |
When comparing the results from samples C2 to C5, it is observed that the combined addition of Ni and Cr in samples C4 and C5 also decreased their CE and VC values respect to C2 and C3 samples. However, these two samples contain pearlite and carbides apart from ferrite while samples C2 and C3 are fully ferritic alloys.
In case of alloys C6 to C9, the addition of Al and the increasing content of this alloying element also decrease their CE and VC values. However, it must be noticed here that the required Al content to obtain a similar effect on these two parameters to that exhibited by Ni is about 20 times lower than Ni content. On the other hand, Al is an alloying element cheaper than Ni and/or Cr though it increases the risk of gas porosity formation (pinholes) and of spheroidal graphite degeneration during solidification of cast irons (
The VC range required in the standard ISO-12944 for low carbon steels exposed to a C3 corrosivity atmosphere for the first year is 200-400 g/cm2·year. Therefore, all alloys included in
The visual examination of the exposed plates did not show any relevant difference among the alloys when corrosion started (
The identification of those compounds present in the oxide layers formed on the exposed plates are carried out in this section. Samples with dimensions 10 × 20 mm were prepared from one plate manufactured with alloys C1, C5 and C9 and then analysed by X-ray diffraction. Similar diffraction patterns were recorded in the three samples which correspond to lepidocrocite [γ-Fe3+O(OH)] as the main phase present in the oxide layers (
The exposed surfaces and the corresponding cross sections of alloys C1, C3, C5 and C9 have been analysed by EDS-SEM. In all cases, the exposed surfaces are covered with lamellar compounds (see
(b) High magnification SEM image which shows a detail of the corrosion products.
When analysing the corrosion products found in the inner part of the oxide layer (cross sections shown in
The spectrum (e) was recorded in the oxide layer formed on sample C1 while the spectrum (f) represents to those registered from samples C3, C5 y C9.
The semiquantitative results obtained from the EDS analyses carried out on the corrosion products present in different zones of the oxide layers and the surrounding areas to the graphite nodules are shown in
Alloy | Scale zone | Fe | O | Si | Ni | Cr | Al |
---|---|---|---|---|---|---|---|
C1 | Surface | 55.86 | 43.01 | 1.14 | - | - | - |
Inner part | 58.12 | 39.54 | 2.35 | - | - | - | |
Around nodules | 59.96 | 41.67 | 2.37 | - | - | - | |
C3 | Surface | 56.23 | 42.00 | 1.13 | 0.62 | - | - |
Inner part | 54.55 | 42.56 | 2.36 | 0.53 | - | - | |
Around nodules | 56.84 | 39.96 | 2.52 | 0.67 | - | - | |
C5 | Surface | 54.12 | 44.71 | 0.69 | 0.53 | 0.11 | - |
Inner part | 64.64 | 3.43 | 2.07 | 0.67 | 1.18 | - | |
Around nodules | 60.86 | 35.79 | 1.87 | 0.71 | 1.25 | - | |
C9 | Surface | 56.76 | 42.45 | 0.79 | - | - | - |
Inner part | 61.90 | 34.83 | 2.83 | - | - | 0.44 | |
Around nodules | 53.33 | 42.35 | 3.85 | - | - | 0.21 |
Additionally, the inner part of the oxide layers and the compounds surrounding the graphite nodules found in samples C5 and C9 show relevant Cr or Al contents, respectively, according to their chemical composition. However, these two elements have been hardly detected in the surface. For alloy C3, the Ni content of the corrosion products formed on the surface and inner areas of the oxide layer is similar.
As it has been mentioned in the experimental section, the electrochemical corrosion tests were performed by triplicate showing high reproducibility in all cases. The evolution of the OCP with time is shown in
All alloys show a similar behaviour with a cathodic branch which is characteristic of a diffusion control, and an anodic branch with a small slope which is related to the high activity of the tested materials. According to these results, the four alloys can be considered as active materials under cathodic control. In this case, the corrosion current density (icorr) is defined by the oxygen diffusion-limited current density (iL) which is estimated from the current density measured in the middle of the cathodic branch.
Alloy | Ecorr (mA vs Ref) | icorr (μA/cm2) |
---|---|---|
C1 | -712 ± 6 | 28 ± 7 |
C3 | -671 ± 16 | 21 ± 5 |
C5 | -657 ± 29 | 29 ± 1 |
C9 | -691 ± 25 | 29 ± 1 |
Although the four polarization curves plotted in
SEM images of the corroded surfaces of samples C1 and C5 after the electrochemical tests are shown in
In sample C5, the matrix composition markedly differs from those found in the other three samples because it contains pearlite and carbides (
Ductile cast iron plates have been manufactured with 9 different compositions in which Ni, Cr and Al have been used as alloying elements with the goal of improving the corrosion resistance of the resulting cast alloys. The three elements show changes in the microstructure of the alloys and also in their behaviour against atmospheric corrosion. The main conclusions obtained are the following:
The addition of Ni and/or Cr promote the formation of pearlite in the matrix. Moreover, only the presence of Cr originates carbides that remain stable together with the pearlite after the annealing treatment. Fully ferritic matrices are obtained in the rest of the alloys after undergoing such thermal treatment.
The atmospheric corrosion tests performed in this work revealed widespread corrosion on the surfaces of all materials tested, with no visually appreciable differences between the alloys used.
The metallographic studies carried out on the plates exposed to the atmospheric conditions showed that the thicknesses of the oxide layers formed after 186 days of testing depends on the chemical composition of the prepared alloys. The lowest oxide layer thicknesses are obtained for the alloys with about 1.0 wt.% Cr and Ni and with 0.15 wt.% Al.
Electrochemical corrosion studies carried out for four alloys in a 0.6 M NaCl solution indicate that they all show a similar corrosion rate characterised by diffusion control and an active anodic branch.
The authors would like to thank the members of TQC Technologies, S.L.U. for their valuable help when making the experimental work.