Galvanic Corrosion of Ductile Cast Iron Coupled with Different Alloys in Synthetic Domestic Waste Water

CRISTIAN SAVIN1, CARMEN NEJNERU1*, MANUELA CRISTINA PERJU1,2, COSTICA BEJINARIU1*, DIANA BURDUHOS-NERGIS1, ANDREI VICTOR SANDU1,2,3 1Gheorghe Asachi Technical University of Iasi, Faculty of Materials Science and Engineering, 67 D. Mangeron Str., 700050 Iasi, Romania 2 Center of Excellence Geopolymer and Green Technology (CEGeoGTech), School of Materials Engineering, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia. 3Romanian Inventors Forum, 3 Sf. P. Movila Str., 700089 Iasi, Romania

The populated residential areas represent important and grave sources of water quality deterioration. The wastewater is first collected in a compact pumping station -such as a plastic sump -and is then pumped by a wastewater pump to a gravity sewer or to another pumping station. There are certain locations where it is possible to convey sewage by gravity to a central treatment facility or storm water is conveyed up to disposal point entirely by gravity. Whereas, in case of large area being served with flat ground, localities at lower elevation or widely undulating topography it may be essential to employ pumping station for conveyance of sewage to central treatment plant. Sewage and storm water is required to be lifted up from a lower level to a higher level at various places in a sewerages system.
Pumping stations are often required for pumping of untreated domestic wastewater, storm water, combined domestic wastewater and storm water, sludge at a wastewater treatment plant, and treated domestic wastewater. Pumping of sewage is also generally required at the sewage treatment plant.
Several types of pumps are used in the sewerage system for pumping of sewage, sewage sludge, grit matter, etc. as radial-flow centrifugal pumps, axial-flow and mixedflow centrifugal pumps, diaphragm pumps, rotary screw pumps or pneumatic ejectors.
For pumping of water using radial-flow centrifugals and axial-flow and mixed-flow centrifugal type pumps normally bronze impellers, bronze or steel bearings, stainless or carbon steel shafts, and cast iron housing is used. For domestic wastewater pumping using radial-flow centrifugals and axial-flow and mixed-flow centrifugal type pumps similar material is used except that they are often made from cast iron, ductile cast iron or stainless steel impellers. Pumping of sewage is different than common water pumping due to polluted nature of the wastewater containing suspended solids and floating solids, which may clog the pumps. The dissolved organic and inorganic matter present in the sewage may chemically react with the pump and pipe material and can cause corrosion. Domestic wastewater consists of relatively weak solutions of non aggressive contaminant chemicals. The types and concentrations of contaminants in raw wastewater from domestic sources are fats, oils, greases, soaps, organic matter, dirt, human waste, food waste and anorganic compounds.
From a corrosion perspective, the sewage is no more corrosive than ordinary water, i.e., water that presents a pH near the neutrality. Wastewater is aerated in most parts of the wastewater system, at least where the biological reactions do not consume all the dissolved oxygen and produces much more corrosive conditions for many metals. The most common chemical contaminants in domestic wastewater are chlorides, nitrogen compounds and a wide variety of organic compounds. Sulfate and phosphate ions are present. The pH of domestic wastewater typically is between 6 and 7, running slightly on the alkaline side of neutral where there is higher use of soaps and household cleansing materials, most of which are mildly alkaline to increase their detergent effectiveness. Used water from manufacturing plants and factories can have a wider range of contaminants -some of which may significantly affect the corrosiveness of the wastewater. Accidentally, wastewaters can achieve low pH (acidic waters) or high pH (alkaline waters) which brings a corrosion accentuation [1][2][3][4][5][6].
Carbon steel and ductile cast iron both corrode slowly in aerated wastewaters. Corrosion rates in the normal aerated wastewater depend on the amount of aeration and to a lesser degree on the temperature. They seldom exceed 0.25 mm/y. This is relatively slow, but is high enough that carbon steel requires continual maintenance to achieve the long service life normally expected from pipes and other wastewater equipment, especially inside wastewater treatment plants. Corrosion rates of carbon steel and cast iron in wastewater can be accelerated 5 to 10 fold by local acidic conditions produced by microbiological action or by accidentally contaminations. Furthermore, pipes and other structure made from carbon steel and ductile cast iron can be externally corroded by the surrounding soil. In some situations it is likely that they will be in contact with more traditional materials as mixed systems where galvanic forces can interfere with the corrosion resistance of the individual metals adding a supplementary corrosiveness [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].
When a metal is immersed in a conducting liquid it takes up an electrode potential (also known as the corrosion potential). This is determined by the equilibrium between the anodic and cathode reactions occurring on the surface and it is usually measured with reference to a standard electrode such as the saturated calomel electrode (SCE) [1][2][3].
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when is in electrical contact with a different type of metal and both metals are immersed in an electrolyte [1][2][3][4][5][6]. When two or more different type of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathode metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded. Even a single type of metal may corrode galvanic ally if the electrolyte varies in composition, forming a concentration cell.
The reactions which occur are similar to those that occur on single, uncoupled metal, but the corrosion rate is appreciable increased. The effect of coupling the two metals together increases the corrosion rate of the anode and reduces or even suppresses corrosion of the cathode.
When a metal is corroding two processes occur. One is the dissolution of metal at the anode. In the case of ferrous alloys Fe is dissolving metal, as: This must be balanced by a cathode reaction. Most practical cases of bimetallic corrosion occur in solutions containing dissolved oxygen and in most neutral and alkaline liquids the primary cathode reaction is the reduction of dissolved oxygen: In acidic liquids the cathode reaction is often the reduction of hydrogen ions to hydrogen gas: In deaerated environments other cathode reactions can occur, as reduction of partially oxidized sulphureous species. Under uncoupled corrosion the anodic and cathodic reactions occur at small, local areas on the metal. In a bimetallic couple the cathodic reaction is more, or totally, on the electropositive member of the couple and the anodic reaction is mostly, or totally, on the electronegative component of the couple.
Many factors can influence the corrosion in a coupled pair of different metals, i.e. couple potential (E couple ) and current density of couple (j couple ). Among these on can evidences: the difference in electrode potentials of the individual partner of the couple, the quality of the electrical connection between the metals, the conductivity of the electrolyte, area ratio of the two electrodes, their configuration (i.e., the distance between the dissimilar metals), aeration, metallurgical composition and stifling effects due to the diminishing rate of diffusion of oxygen through the electrolyte and through films of corrosion product to cathode regions. Similar studies were performed on different type of materials, according to their use [21][22][23][24][25][26][27][28][29][30][31][32].
This investigation was aimed to study the effect of coupling the ductile cast iron (nodular cast iron) with different alloys in three type of synthetic wastewater.

Experimental part
The ductile cast iron was coupled cu different alloys wherewith can come into contact in pumping stations. The compositions of the ductile cast iron and those of the other alloys used in this study are presented in table 1. These compositions were determined by optical emission Synthetic test waters were prepared according with the Boeije [7] studies concerning the mean composition of the settled water from the sewage of the municipal sewerages. The constituent selection of the synthetic solutions is based on the expected composition of domestic waters exhaust. Some changes were made in the Boeije composition, regarding the ratio between inorganic constituents especially. The composition of the basic synthetic wastewater, denoted DWW-1, is presented in table 2. The inorganic components are respondent for the corroding properties of the solution, while the alimentary components behave as corrosion inhibitor or protector. This solution has a pH closed to neutrality: pH=6.5.
Towards analyze the pH influence regarding corrosion behavior, the pH of the base solution (DWW-1) was modified by adding hydrochloric acid and sodium hydroxide respectively. Thus were obtained the alternative solutions: DWW-2, with pH=3.0, by titration of basic solution with 0.1 M HCl solution; DWW-3, having a pH=11.0, by titration of basic solution with 0.1 M NaOH solution.
To evaluate the galvanic corrosion parameters, galvanic potential -E couple and galvanic current -j couple , the Evans method and a commercial potentiostat/galvanostat were used. All measurements were performed with the VoltaLab 21 (PGP201) potentiostat (Radiometer Analytical SAS -Franþa), equipped with the acquisition and processing data software VoltaMaster 4 [33][34][35][36][37]. A special three electrodes electrochemical cell was used. This cell is presented in (fig.1). Ductile cast iron was mounted as work electrode, while the other materials were mounted as auxiliary electrode. A saturated calomel electrode (SCE) was used as a reference. All electrodes were realized as cylindrical disks (10 mm diameter and 2 mm thickness), and were screwed at cell connectors.
The free surface of the connector was covered with acrylic glass. In these conditions, all electrodes present the same surface: 2.76 cm 2 .
Each specimen was polished with SiC paper, gradually, down up 2000 grit specification, degreased with acetone, washed in distilled water, and maintained in working solution for a determined period of time, up to constant temperature. The measurements were performed in fresh natural aerated solutions at 23 o C.
The measurements were performed according to Evans method using the facilities offered by VoltaLab 21 potentiostat and VoltaMaster 4 acquisition software, using Evans test sequence (Coupled Corrosion -Evans) respectively. Briefly, an imposed current is applied gradually to the two electrodes and the voltammetric curves are registered, namely: potential of the working electrode E work versus current and potential of the auxiliary electrode (E aux ) versus the same current.
Both potentials are measured versus reference electrode. The measurement stops when E work -E aux potential sign reverses. The intersection of the two curves determine the corrosion current and mixed potential (named and rest potential). As an example, in figure 2 is illustrated a typical diagram for ductile cast iron-stainless steel couple in DWW-1. To compare the data obtained in different situations, instead of the corrosion current (I) the corrosion current density (j = I/S) is calculated.

Results and discussions
Four or six successive measurements were performed for each pair of coupled alloys. The mixed potential and the couple current densities are presented in tables 3, 4 an 5, dependent on corrosion medium and elapsed-time from the measurement starting. In all measurements the aria ratio of the anode and cathode were strictly equal with unity and the system configuration was unchanged, so that these parameters no influence actual results.
Very different results were obtained depending on nature of coupled alloys and corrosion medium pH. Some conclusions are presented forwards.
In the case of Ductile Cast iron/Stainless steel couple, the corrosion potential for the couple increase slightly with immersion time in all three synthetic waters and tend to a constant value. In alkaline water the corrosion potential is moved towards more negative value. Current densities of the couple have little and constant values in quasi neutral waste water (DWW-1) and are of approximately eight times higher in acidic waters (DWW-2). In acidic water the couple current density decrease in time, due to the appreciable increase of the corrosion products layer at high value of couple current. In alkaline waste water (DWW-3) the couple current density is of only two times higher than in neutral waters and increase with time.
The couple Ductile cast iron/Bronze behaves approximately likewise in the three corrosion media, the current density of couple being comprised between 60 and 80 µA/cm 2 , saving that in DWW-1 the current density decrease from 60 µA/cm 2 till 45 µA/cm 2 in 45 min. The corrosion potential for couple also increase slightly in time, a more pronounced increase being evidenced in neutral water.
In all the three corrosion media, for Ductile cast iron/ Brass couple the current densities decrease with time immersion, the decrease being also more pronounced in DWW-1. In acidic and basic media the current densities The coupling of Ductile cast iron with Cast metal care is less unfavourable in the three corrosion media; in DWW-1 the corrosion density decrease rapidly from 22 to 10 µA/ cm 2 (16 min), in DWW-2 the corrosion density increase from 20 µA/cm 2 to 30 µA/cm 2 (14 min) and in DWW-3, the corrosion density is practically equal with 6 µA/cm 2 (the most advantageous couple). The increase of corrosion current in acidic media is probable due to the solubility of the corrosion products.   The Ductile cast iron/Aluminium couple shows a different behavior. Thus, in neutral and basic waters the current densities are very small and constant (6 µA/cm 2 in DWW-1 and 10 µA/cm 2 in DWW-2), this couple being harmless. In these couplings Aluminium is more noble partner and Ductile cast iron the less noble partner. In acidic medium (DWW-2) Ductile cast iron is more noble partner while Aluminium corrodes with enough high speed.
Taking into consideration only the initial values (at t=0 min) of the couple current density, the following galvanic series can be determined: Least noble -anodic Most noble -cathodic In DWW-1: DCI < AQL < HSLA < CI < SS < BRZ < BRS In DWW-2: AL < DCI < CI < HSLA < BRZ < BRS < SS In DWW-3: DCI < CI < AL < SS < HSLA < BRZ < BRS Thus, the coupling of Ductile cast iron with Brass, Bronze or Stainless steel appear to be the most disadvantageous arrangement.
Acidic waste water enhances the galvanic corrosion of the analyzed of alloy pairs.