The Preparation and Performance Study of a Phosphate-Free Corrosion/Scale Inhibitor

By using acrylic acid copolymer, sodium citrate, hydrolyzed polymaleic anhydride (HPMA), corrosion inhibitor D and Zn synergist as raw materials, a multi-component phosphate-free corrosion and scale inhibitor was developed. The performance of the composite phosphate-free corrosion and scale inhibitor was evaluated using the rotary hanging sheet corrosion test, the static scale inhibition test and the corrosion electrochemical test. And the surface morphology of the carbon steel was observed by scanning electronic microscope (SEM). Orthogonal experiment results indicated that the optimal mass ratios of amino acid: Zn synergist: HPMA: corrosion inhibitor D: acrylic acid copolymer was 0.5:10:12:1:8. It was also observed that phosphate-free corrosion and scale inhibitor based on an anodic reaction through the electrochemical corrosion experiment, its annual corrosion rate and scale inhibition rate reached 0.0176 mm·a and 98.3%, respectively, showing excellent corrosion and scale inhibition performance.


Introduction
Industrial recirculating cooling water accounts for 60% -70% in the industrial water, it will result in corrosion, scaling and microbial slime of the equipment and the pipeline as well as reduce their service life without treating.Thus, to treat recirculating cooling water has aroused the concern from the whole society.
Organophosphorus compound was a type of corrosion inhibitor extensively used since 1980s.However, it's diminishing because of its toxic effects on aquatic and other life [1].Therefore, the public began to focus on the development of low phosphorus, non-phosphorus and heavy nonmetal salt corrosion and scale inhibitors.On one hand, low phosphorus corrosion and scale inhibitor is developing rapidly, such as 2-phosphono-1,2,4-tricarboxylic acid butane (PBTCA) and hydrolyzed polymaleic anhydride (HPMA) [2].On the other hand, the non-phosphorus corrosion and scale inhibitor has also obtained new achievements.
In this paper, a multi-component phosphate-free corrosion and scale inhibitor blend composed of acrylic acid copolymer, sodium citrate, hydrolyzed polymaleic anhydride (HPMA), corrosion inhibitor D and Zn 2+ synergist was prepared [3][4][5][6][7].The recirculating cooling water was obtained from water treatment plant.The performance of the phosphate-free corrosion and scale inhibitor was eva-luated by rotary hanging sheet corrosion test and static scale inhibition test.The mechanism of corrosion and scale inhibition was preliminarily investigated by corrosion electrochemistry test and scanning electron microscope (SEM) techniques [8][9][10][11].

The Rotary Hanging Sheet Corrosion Test
The corrosion rate of A3 type carbon steel was determined by weight loss test.The carbon steel sheets were polished with different grades of emery paper, degreased with anhydrous alcohol, and rinsed with distilled water.Having been dried by electric dry oven and accurately weighed by electronic balance, the carbon steel were immersed in a beaker with water sample and with or without inhibitors.And the quality index of water sample was presented in Table 1.The beakers were put into the rotary hanging piece corrosion test instrument.Experimental conditions as follows: 1) constant temperature was 45˚C ± 1˚C; 2) rotating speed was 75 r•min -1 ; 3) The annual corrosion rate and corrosion inhibition efficiency was calculated by formulae (1) and ( 2), respectively [12]: where K a is the annual corrosion rate, mm•a -1 ; m 0 is the weight of the carbon steel before experiment, g; m is the weight of the carbon steel after experiment, g; s is the surface area of the carbon steel, cm 2 ; ρ is the density of the carbon steel, g•cm -3 ; t is the test time, h.
where K is the corrosion inhibition efficiency; K 0 and K 1 are the values of the weight loss of carbon steel after being immersed in solutions without and with inhibitors, respectively, mm•a -1 .

The Static Scale Inhibition Test
The mechanism of scaling inhibition mainly included lattice deformation, complexation, dispersion effect and dispersive active [13,14].In the experiment, 0.5 L•min -1 of nitrogen was ventilated to the bottom of the container with adding 30 mg•L -1 of inhibitor at 45˚C.The experiment was ceased after six times diluted concentration.
where C 2 is determined mass concentration of calcium ion after adding chemicals, mg•L -1 ; C 1 is determined mass concentration of calcium ion without chemicals, mg•L -1 ; C 0 is initial mass concentration of calcium ion without chemicals, mg•L -1 ; K is the concentration multiple in the determined conditions of C 2 and C 1 .

.Corrosion Electrochemical Experiments
Corrosion electrochemical methods consulted the research methods and measurement techniques in other fields.According to the characteristics and requirements of corrosion metal electrodes, it has become a significant method for the research and measurement of corrosive electrochemistry with improvement and modification.In particular, the dynamic potential polarization is the common method that evaluates inhibition efficiency of corrosion inhibitor [15].Polarization curve obtained from the dynamic potential polarization extrapolates corrosion potential to obtain corrosion current density I and Tafel parameters.The shape of cathode and anodic polarization curve intuitively reflects the corrosion inhibition mechanism of corrosion inhibitors, which is the main research and evaluation method of corrosion inhibitors.Electrodes were connected by three electrodes polarization method in the experiment.A3 carbon steel was used as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum sheet as the auxiliary electrode.The sample was immersed in a 3.5% (w/w) NaCl solution for 30 min.When polarization curve test was carried out, the potential scan rate was adjusted to 0.01 v•s -1 .Polarization curves could be achieved after data process.

Scanning Electronic Microscopy (SEM)
Corrosion crystal morphology on the surface of A3 carbon steel was observed by JSM5610LV type SEM.In this experiment, A3 carbon steel was immersed in water sample with and without phosphate-free scale and corrosion inhibitor, respectively.Afterwards the test coupon was washed with 98% (w/w) anhydrous alcohol and treated for vacuum drying.Accelerating voltage of the JSM5610LV type SEM was 25 kV, amplification factor was 2000.

Rotating Hang-Parcel Weight-Loss Experimental Performance Analysis
The inhibitor in this experiment composed of A (sodium citrate), B (Zn 2+ synergist), C (HPMA), corrosion inhibitor D and E (acrylic copolymer).L16 (4 5 ) orthogonal table was used to arrange experiment.Corrosion rate of the carbon steel was determined after 72 hours, the final result was the average values of three replicates.The re-HCO sults were showed in Table 2. sion inhibition effect.As shown in Table 2, the sequence of the influencing factors was corrosion inhibitor D > Amino acid > Zn 2+ synergist > acrylic copolymer > HPMA in order of importance to the corrosion rate.According to orthogonal experiment results, the optimum formula was D 4 A 4 B 4 E 1 C 1 on corrosion inhibition.Based on the optimum formula, the weight loss experiment was carried out.The mass ratios of amino acid, Zn 2+ synergist, HPMA, corrosion inhibitor D and acrylic copolymer was 0.5:12:0:1:8 in the optimum formula.The results were presented in Table 3.

Static Scale Inhibition Performance Analysis
Comparison experiments were conducted with and without inhibitors.The relationship between the concentration of Ca 2+ , 3  and cycle of concentration was showed in Figure 1.
When the solution concentration was concentrated by 1 -1.8 times, the concentration of Ca 2+ and 3 HCO  showed linear change, illustrating that Ca 2+ and 3 HCO  were unsaturated in solution.The concentration of The corrosion inhibition efficiency of optimized formulation reached 96.6% (Table 3), showing good corro-3 HCO  appeared increases linearly with the inhibitor,  while it increased slowly and even tended to decrease without the inhibitor.And the concentration of Ca 2+ basically showed increases linearly with and without inhibitor, respectively.Experimental results showed that the concentrations of Ca 2+ and in the solution were stabilized after adding inhibitor.
According to the results of Table 4, the average scale inhibition efficiency with phosphate-free corrosion and scale inhibitor reached 98.3%, showing good scale inhibition performance.

Corrosion Electrochemistry Results Analysis
Two specimens were immersed in 3.5% NaCl solution with and without inhibitor, respectively.Then the test method of electrochemical polarization curve was conducted.The results were presented in Table 5 and Fig In Figure 2, the corrosion potential was shift to the positive after adding the inhibitor.The increasing in anodic current density indicated that the dissolution of the anode materials.Figure 2 showed the anode I corr was reduced from 201 μA•cm 2 to 21 μA•cm 2 after adding the inhibitor.It indicated that the anode corrosion process was inhibited.And the anode inhibition was more significantly than the cathode with the inhibitor.Table 3 and Table 5 showed the consistent results with electrochemical test.

SEM Structure Analysis
The experimental conditions as followed: the bath temperature was maintained at 50˚C for 48 h, the flow rate of solution and the cycle of concentration were adjusted to 0.01 m•s -1 and 1.8 times, respectively.The concentration of phosphate-free corrosion and scale inhibitor was 30 mg•L -1 .Figures 3(a) and (b) were corrosion crystal on A3 carton steel surface with and without phosphate-free corrosion and scale inhibitor, respectively.
Figure 3 showed that the carbon steel was corroded severely without the inhibitor, many apertures appeared on its surface and some corrosion products deposited on the surface with uplift shape.In comparison, the surface of the carbon steel with 30 mg•L -1 of phosphate-free   corrosion and scale inhibitor was flat and smooth.It caused some damage and few corrosion products accumulation on the surface of the carbon steel because of polishing hang-parcel.But there were basically no corrosion marks.Results illustrated that a complete compact protective film formed on carbon steel surface after adding phosphate-free corrosion and scale inhibitor.It effectively cut off contact between the corrosive medium and carbon steel surface so as to significantly inhibit the corrosion of carbon steel in water sample.
2) Annual corrosion rate tested by optimized rotating hang-parcel corrosion experiment was 0.0176 mm•a -1 which was lower than 0.075 mm•a -1 of industry standard according to Code for Design of Industrial Recirculating Cooling Water Treatment (GB50050-2007).The experimental results showed that the phosphate-free corrosion and scale inhibitor was a kind of good corrosion and scale inhibitor whose corrosion inhibition rate and scale inhibition rate surpassed 96% and 98%, respectively.
3) Polarization curve test showed the phosphate-free corrosion and scale inhibitor was a kind of corrosion and scale inhibitor that mainly caused anode type reaction.Based on the surface morphology of the carbon steel was observed by SEM, it showed that a protective film was formed on the surface of A3 carbon steel with optimized phosphate-free corrosion and scale inhibitor.

- ure 2 .
And the inhibition efficiency of the inhibitor for the carbon steel corrosion was calculated by formula(4

Figure 2 .
Figure 2. The polarization curve of blank and adding phosphate-free scale corrosion inhibitor treated samples in 3.5% NaCl solution.

Figure 3 .
Figure 3.The corrosion crystal on A3 carton steel surface.