Environmental-friendly electrolyzed water and its application for metal surface treatment
Toshikazu Takenouchi and Yasuaki Nishio**JIPCPM LLC
Abstract
Anode water (acidic electrolyzed water, AcEW) and cathode water (alkaline electrolyzed water, AlEW),
which are produced from dilute electrolyte solutions by electrolysis, are referred to as electrolyzed water (EW).
In this review, the principles of generation of EW and characteristics of AcEW and AlEW are described; andits use and application for surface treatment of metals are introduced. AlEW can be used for degreasing andrinsing metal surfaces, which shows equal precision cleaning performances compared to conventional methods
that use detergents. AcEW can be used for removing and etching oxidized layers of metal surfaces androughing of metal surfaces. The use and application of EW for surface treatment of metallic materials arepromising.

1. Introduction
Electrolyzed water (EW) is a generic name for aqueous solutions produced from dilute electrolyte solutions
by electrolysis [1]. EW has been used in the medical and hygiene fields and has been increasingly used in theindustrial fields in recent years. In the past, organic solvents were used for degreasing metal parts; however,recently aqueous cleaning agents consisting primarily of surfactants replaced them. Furthermore
environmental-friendly alkaline electrolyzed water (AlEW) is used as non-toxic and easy-to-use cleaning
agents instead of surfactants. Meanwhile inorganic acids, such as hydrochloric acid and sulfuric acid, havebeen used for the removal of rust on the metal surfaces; and acidic electrolyzed water (AcEW) is used as analternative. In this review, I illustrate the characteristics of EW and describe its application for metal surface
treatments, such as degreasing, rinsing, removal of oxidized layers and etching metallic materials, using its
characteristics.

2. The generating principles and characteristics of EW
Anode water and cathode water are generated from an electrolyte solution in an anode chamber and acathode chamber, respectively, by electrolysis using a diaphragm type electrolytic device. Sodium chloride(NaCl) is commonly used as an electrolyte for the generation of electrolyzed water. In the anode chamber,
oxygen and hydrogen ions are generated by the oxidation of water, and chlorine by oxidation of chloride ions.
Chlorine reacts with water to yield hypochlorous acid (HOCl) and hydrochloric acid (HCl). As a result, anodewater is acidified; and its oxidation reduction (redox) potential (ORP) increases significantly. Meanwhile, in a1cathode chamber, water and dissolved oxygen are reduced to yield hydrogen and hydroxide ions; and cathodewater becomes alkaline. Cathode water shows an increased dissolved hydrogen concentration and a decreased
dissolved oxygen concentration simultaneously; and its ORP decreases significantly. Reactions that occur at
the anode and the cathode during electrolysis are shown as equations (1) – (5). When sodium sulfate (Na2SO4)
is used as an electrolyte, chlorine is not generated in the anode chamber; and only equation (1) occurs,
resulting in an elevated dissolved oxygen concentration.
[Anode]H2O  1/2O2 + 2H+ + 2e-… (1)2Cl-  Cl2 + 2e-…(2)

Cl2 (aq) + H2O  HCl + HClO
[Cathode]-

2H2O + 2e  H2 + 2OH
-

-

…(3)
…(4)

1/2O2 + H2O + 2e  2OH

-

…(5)

The characteristics of AcEW and AlEW were obtained using NaCl or Na 2SO4 as electrolytes (referred to as
NaCl AlEW, AlEW or Na2SO4 AcEW, AlEW) and are shown in Table 1. The ORPs of AcEW are higher thanthose of acid solution at the same pH; and those of AlEW are lower than those of the alkaline solution at thesame pH. NaCl AcEW shows an elevated ORP because of the formation of chlorine and hypochlorous acid.
Na2SO4 AcEW, which consists of no chlorine, shows an elevated ORP because of an increased dissolved
oxygen concentration. AlEW shows a negative ORP that is lower than that of sodium hydroxide (NaOH)
solution at the same pH. The characteristics of EW are as follows: HOCl, which is thermodynamically unstable
in aqueous solutions, can be formed by electrolysis; acidic and alkaline solution can be formed
electrochemically without using dangerous acids or alkaline; and therefore the work procedure is safe.

Table 1

2.5
2.5

Redox Potential
mV vs.SHE
850
1300

Dissolved Oxygen
mg/dm3>20
15

Dissolved Hydrogen
mg/dm300Free chlorine
mg/dm3
0
20-60

2.5

750

8

0

0

11.5
11.5
11.5

-450
-450
290

4
4
8

1.5
1.5
0

0
0
0

pH
Na2SO4-AcEW
NaCl-AcEW
Sulfuric acid
Hydrochloric acid
Na2SO4-AlEW
NaCl-AlEW
Sodium hydroxide

Characteristics of electrolyzed water

An electrolytic cell to generate EW consists of an anode chamber and a cathode chamber, which areseparated by means of a diaphragm (diaphragm type electrolytic cell). Electrodes are made of titanium coatedwith platinum. In the industrial field, the diaphragm and flow type electrolytic apparatuses that have ageneration capacity of 2 – 3 L/min are commonly used. The appearance of the apparatus [2] is shown in Figure
1; and the conceptual diagram of flow type electrolysis in Figure 2. AcEW generated from a typical2electrolytic apparatus has a free chlorine concentration of 30 – 50 mg/L. Demerits of AcEW are a generation ofchlorine gas at anode during electrolysis which causes metal to corrode and a deterioration of working
environment by smell of chlorine. Improvement of these demerits is accomplished by high-voltage electrolysis[3]. It adapted an increased current density so that the NaCl concentration can be decreased to as low as 0.01 %,resulting in a free chlorine concentration of 5 mg/L. Because chlorine gas is not generated, the corrosion of
metal and the working environment are improved, so this electrolytic apparatuses are frequently used for
degreasing, rinsing and surface treatment of metallic materials for precise electrical parts [4, 5].

Fig. 1 EW generating system (Espax, JIPCM [2] )Fig. 2 Conceptual diagram of flow type electrolysis

3. Potential-pH diagram for the Fe-H2O system
Regarding the stability of metal in pure water, Pourbaix diagrams [6] are useful. Potential-pH for the six
solutions (shown in Table 1) is superposed on a potential-pH diagram for the Fe-H2O system (Figure 3).
A dashed line a represents an equilibrium potential for oxygen generation, which is expressed as EO2 = 1.23 –0.059pH. A dashed line b represents an equilibrium potential for hydrogen generation, which is expressed as
EH2 = – 0.059pH. Water is stable within a region between line a and line b; oxygen is generated at a higher
potential than line a; and hydrogen is generated at a lower potential than line b. 1 in Figure 1 represents
potential-pH for the NaCl AcEW, 2 for Na2SO4 AcEW, 3 for hydrochloric acid and sulfuric acid at pH 2.5, 4
for NaOH solution at pH 11.5, 5 for NaCl AlEW, and 6 for Na2SO4 AlEW, respectively (for the potential-pH
for 1 – 6, refer Table 1).
The potential-pH for AcEW lies on the corrosion region for iron, therefore iron dissolves. Meanwhile thepotential-pH for AlEW lies on the passivity region for iron, therefore iron does not corrode. Even though ORP
is low, corrosion occurs at a pH above 14. NaCl and Na 2SO4 AlEW consist of chloride ions and sulfate ions,3

respectively; and when iron is immersed in those solutions at low ion concentrations corrosion is less likely tooccur. When iron is immersed in Na2CO3 AlEW corrosion does not occur.

Fig. 3 Pourbaix diagrams of iron superposed with potential-pH for the six solutions.
Refer the number 1 – 6 to table 1.

4. Characteristics of AlEW and its use
4.1. Degreasing
One of the characteristics of AlEW is degreasing action. Degreasing oxygen-free copper which wascontaminated with machine oil was tested. AlEW, NaOH solution at the same pH as AlEW (11.5), and purewater were used as cleaning liquids; and the metal was spray-washed with heated cleaning liquids followed bya rinsing with pure water then drying. The cleaning properties of those cleaning liquids are compared with thatof organic solvent and surfactant, which are used in conventional specifications. The results are shown in
Figure 4. Degreasing with AlEW shows that the residual oil content and residual ion content are equal to thoseof conventional specifications, suggesting it is able to apply precision cleaning [7]. The result of surface
analysis shows that oxidation of oxygen-free copper does not occur by degreasing with AlEW.
The mechanism of degreasing metal with machine oil by AlEW appears to be the following: grease ishydrolyzed by alkaline to yield soap, which acts as a surfactant and form micelles to emulsify mineral oil thatis hydrocarbons. Furthermore spraying heated AlEW plausibly removes oil layer by the pressure of spraying
because of lowered oil viscosity by heat, resulting in an increased cleaning property. In comparison with
NaOH solution to AlEW at the same pH, the latter shows an increased cleaning property. The degreasing4mechanisms of AlEW appear to consist of saponification by alkaline and pressure of spraying, as well ashydrogen nano-bubbles [8] generated by electrolysis, which may increase the degreasing efficiency.
The higher the pH of AlEW, the more efficacious is its’ degreasing effect; and therefore a generation deviceof AlEW at pH of more than 12 is commercially available. However, such AlEW is associated with some risks
due to a high pH, so AlEW may be used at a pH of less than 12 in light of safety. Alternatively, there can be amethod to use diluted AlEW at a pH of 13, which is stored in a plastic container, and used as needed; however,stored AlEW, in which hydrogen nano-bubbles disappear, has the same characteristics with the NaOH solution
at the same pH; and therefore the effect of hydrogen nano-bubbles cannot be expected. Furthermore, AlEW at
a pH of 13 is also associated with some risks due to a high pH.

Fig. 4 Comparison of residual oil content and residual ion content
on the copper surface by several cleaning liquids

4.2. Rinsing
In addition to the degreasing effect, AlEW has a rinsing effect, that is, an effect to remove fine particles andsurfactants from the surface of cleansed objects [9]. The rinsing effect is attributed to the alkalinity of AlEW,
as well as hydrogen nano-bubbles that enhance the cleansing effect on the cleansed objects and the fineparticles on their surface. It is explained that hydrogen nano-bubbles in AlEW adhere onto the fine particles,
resulting in an induction of a physical peel force to separate them from the cleansed objects [10]. Furthermore,
the surface potential of fine particles has a large negative value in alkaline solutions, so it is explained that5

detached fine particles repel each other, as well as to the cleansed objects, and disturb re-adherence to thesurface [11]. It is explained that the effect of AlEW on removal of anion surfactants is attributed to hydrogennano-bubbles by the same mechanism as the removal of fine particles. It is speculated that when cleansed
objects with remaining anion surfactants are immersed in AlEW, hydroxyl ions adhering on hydrogen nanobubbles surfaces are selectively substituted to anion surfactants via an ion exchange reaction, as a result, anion
surfactants on the cleansed objects are detached. It is explained that detached anion surfactants repel tonegatively charged hydrogen nano-bubbles, and thus do not re-adhere to the surface of cleansed objects,
resulting in the efficient rinsing and washing. A pattern diagram of rinsing is shown in Figure 5. In this way, it
is reported that the use of AlEW significantly reduces the amount of pure water usage for rinsing whensurfactants are rinsed [12]. In case of degreasing, AlEW can be re-used until its pH decreases; however, in caseof rinsing, AlEW is effective without recycling because the effect is due to hydrogen nano-bubbles. A similar
phenomenon occurs when rinsing sulfuric ions, which tend to remain on the electro-plated surfaces.

Fig. 5 A pattern diagram of rinsing surfactants by hydrogen nano-bubbles.

More than ordinary rinsing using pure water, AlEW of dilute potassium carbonate aqueous solution exhibitsa stronger rinse effect for elimination of remaining sulfate ions when rinsing nickel-plated surfaces. This rinse
effect was recognized even for AlEW that was used 24 h after it was produced, but not 1 week after. Behaviors
of hydrogen nanobubbles observed by dynamic light scattering revealed nanobubbles of about 128-nm
diameter even 24 h after generation. The Ostwald ripening phenomenon was observed. Hydrogen nanobubbles
in an open system changed: some shrank because of ripening, later dissolving in the aqueous solution anddisappearing; others showed swelling and expansion. One week later, few nanobubbles smaller than 300 nm
were observed. Rinse effects by AlEW, which are attributable to the actions of hydrogen nanobubbles
generated in AlEW, occur because sulfate ions are cleaned and removed from the nickel-plated surface [13].

64.3. Cutting
When metal materials are cut, cutting fluids are generally used. However, because cutting fluids consist ofvarious ingredients they are collected and disposed by waste disposal companies. And cut surfaces arecontaminated with cutting fluids, so the metal materials should be cleansed with organic solvents. However,organic solvents are highly volatile and have adverse effects on environmental health, so a reduction in use oforganic solvents is required. Recently, in order to resolve these problems, AlEW has been used as analternative to cutting fluids [14 – 16]. AlEW has a superior cooling effect compared to oil; and thus the cutting
temperature can be kept low. Furthermore, AlEW can prevent high-temperature oxidation and corrosion of
cutting surfaces, and has a distinct advantage of not corroding the cutting facility. In addition, AlEW has adegreasing effect as described in the section 4.1., so the cutting surface that is cut with AlEW has an
unadulterated surface. AlEW used for cutting is generated from potassium carbonate as an electrolyte and does
not consists of corrosive ions, so metal does not corrode even when left unwashed. Waste of AlEW after use
can be disposed of as it is after oil-water separation, suggesting that it may reduce the production costs andenhance the production efficiency.
Occasionally, grease can be added to AlEW to improve the lubricating effect. Grease is saponified by AlEW
to yield soap and glycerin, which enhance the lubricating effect. A cutting fluid based on AlEW added withgrease has a less greasy content than conventional aqueous cutting fluids and does not consist of otheringredients like anti-corrosion agents, suggesting that it makes the waste water treatment easier. It is expectedthat AlEW will be used as a cutting fluid from the viewpoints of improvement of cleanliness of cutting
surfaces and a reduced cost of liquid waste treatment for cutting fluids. In addition, AlEW at a pH of 12 – 13 issometimes used in mixtures to prevent cutting fluids from emitting a putrefactive odor. It is due to the death of
microorganisms that cause the putrefactive smell at a pH of more than 12. AlEW of sodium silicate is used forprocessing and degreasing aluminum materials [17]. Aluminum is an amphoteric metal and corrodes in bothacidic and alkaline solutions; however, it does not corrode in AlEW of sodium silicate. It may be attributed to
the formation of passive layers of silicate salts on the aluminum surface.

5. The characteristics of AcEW and its application
5.1. Removing oxidized metal layers
AcEW, which has the same effects of acid, is able to dissolve and remove oxidized layers of metal and to
etch the under layers of metal surfaces. Oxygen free copper was heated in the atmosphere to form thermallyoxidized layers and used as test specimens, followed by the immersion in NaCl AcEW and hydrochloric acidsolution at the same pH; and then the change of their surfaces was studied by Auger electron stereoscopy
depth-profile elemental analysis. As a result, both specimens showed a significantly reduced intensity ofoxygen, suggesting that thermally-oxidized layers were removed. The oxidation state of the underlying surface

7

of treated oxygen free copper whose thermally-oxidized layers were dissolved and removed is nearly equal tothat of surface of oxygen free copper that has naturally-oxidized layers.
Iron specimens were prepared as the same way, immersed in NaCl AcEW and surface analysis was
performed [18]. The result showed that thermally-oxidized layers were dissolved and removed and theoxidation state of surface of treated iron is nearly equal to that of surface of iron without thermal treatment.
The SEM (Scanning Electron Microscopy) images of iron specimens before and after the immersion of NaCl
AcEW are shown in Figure 6. The result confirmed that the surface of iron whose thermally-oxidized layers
were removed is finely roughened.

Fig. 6 SEM images of iron surface as sintered before immersion (b),and dissolved oxidized film after immersion in NaCl AcEW (a).

The dissolution and removal of oxidized layers were also observed using Na 2SO4 AcEW, as well ashydrochloric acid and sulfuric acid at the same pH. Because the oxidation state of the surface of underlying
copper and iron whose thermally-oxidized layers were dissolved and removed by AcEW is nearly equal to thatof surface of copper and iron that have naturally-oxidized layers, it is suggested that AcEW can be used for
pretreatment, such as scaling and removing rusts, as an alternative to hydrochloric acid and sulfuric acid.

5.2. Metal etching
Copper foils were electro-plated with copper 1 µm in thickness and used as test specimens. They were
immersed in NaCl AcEW and hydrochloric acid at pH of 2.5; and their appearances were studied by SEM [19].
The results are shown in Figure 7.
When specimens were immersed in hydrochloric acid, dissolution was observed from the crystal grainboundary, however in NaCl AcEW a significant pitting corrosion occurred. Nickel plates were studied as thesame way, and as shown in Figure 8 a significant pitting corrosion occurred in NaCl AcEW.

8

Fig. 7 SEM images of copper electro-plated surface before immersion (a),immersed in pH 2.5 HCl solution (b), and in NaCl AcEW (c).

Fig. 8 SEM images of nickel plate surface before immersion (a),immersed in pH 2.5 HCl solution (b), and in NaCl AcEW (c).

As just described, when NaCl is used as an electrolyte, free chlorine generated in anode etches metal
surfaces to form convex-concave surfaces (pitting corrosion). Roughening metal surfaces is possible using
pitting corrosion by free chlorine in NaCl AcEW. SEM images of electrolytic copper plating surfaces that issprayed with NaCl AcEW are shown as examples, in Figure 9. The images show that convex-concave surfaces
are uniformly formed. The increase in adhesiveness of electro-plated films coated on the surface may be
attributed to roughening metal surface.

Fig. 9 Surface morphology of electro-plated copper (a) and copper plate (b)etched with NaCl AcEW of 40℃ by spraying.

9

Although Na2SO4 AcEW does not roughen a metal surface like NaCl AcEW, it shows a higher metal
dissolution rate than sulfuric acid at the same pH. Figure 10 shows that the dissolution rate of copper in
Na2SO4 AcEW is three times more than those in sulfuric acid and hydrochloric acid. This phenomenon may be
due to the oxidation by dissolved oxygen, which is oversaturated in Na 2SO4 AcEW.

Fig. 10 Dissolution rate of copper immersed in (a) NaCl AcEW,(b) Na2SO4 AcEW, (c) pH 2.5 HCl, or (d) pH 2.5 H2SO4.
As discussed above, AcEW is more oxidative solution and has a higher dissolution rate of copper comparedto hydrochloric acid and sulfuric acid at the same pH. This characteristic suggests that AcEW has a possibilityof the use for etching metal surface.

5.3. Application for circuit boards
AcEW has functions described in sections 5.1 and 5.2. Because of those functions, it is used in the
manufacturing processes of circuit boards (substrates for electronic components with copper circuit wiring)and copper clad laminated boards (substrates without copper circuit wiring) as described below, and has aneffect on their cleaning.
1) dissolution and removal of ground copper fine powder after mechanical polishing of copper cladlaminated board surfaces2) dissolution and removal of finger prints and oxide films on copper clad laminated board surfaces3) rinsing residual chemicals after chemical polishing with a mixture of sulfuric acid and hydrogen
peroxide solution4) minute roughening for copper surface alternative to conventional mixture of sulfuric acid andhydrogen peroxide solution5) removal of development residues of photopolymer for copper circuit wiring formation
6) cleaning electro-plated surfaces

10The use of AcEW in the process of photopolymer development residue is shown in Figure 11 and thetreatment tank for AcEW in Figure 12. Recently, an apparatus with AcEW in the copper circuit wiringformation process of the circuit board [20] has been achieved.

Fig. 11

Process of photopolymer development used by AcEW.

Fig. 12 A treatment tank for AcEW in the process of photopolymer development residue.

6. Conclusions
In the degreasing process for metal materials, degreasing with AlEW achieve the same cleanness equal toother conventional methods with cleansing agents and can be applied to precision cleaning. Furthermore,
AlEW is able to rinse surfactants and fine particles, and this rinsing activity is due to hydrogen nano-bubbles
in AlEW. Meanwhile, AcEW is able to remove oxidized layers and etching and can be used as an alternative
for pretreatments using inorganic acid and is applicable to other surface treatments. In addition, Na 2SO4 AcEW

11

has a higher dissolution rate than sulfuric acid at the same pH; and NaCl AcEW roughens metal surfaces by
free chlorine. Using the forgoing characteristics, AcEW achieved satisfactory results in the application of
circuit boards. AcEW and AlEW is promising in a wide variety of use and application in manufacturing
processes for precision electronic parts as in Figure 13.

Fig. 13 Utilization and application of electrolyzed water for manufacturing process.

References[1] A comprehensive Guide to Electrolyzed Water, Functional Water Foundation, Tokyo (2001)[2] http://www.jipcom.jp/[3] J. Ito and M. Nakamura: Japan Patent No. 260226 (2001）[4] J. Ito and K. Watanabe: Japan Patent No.3363248 (2002）[5] J. Ito and Y. Nishio: Japan Patent No. 4009590 (2007）[6] M. Pourbaix ： Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association ofCorrosion Engineers, Huston (1974)[7] T. Takenouchi, H. Tanaka, and S. Wakabayashi: J surf Finishing Jpn, 54, 818 (2003)[8] K. Kikuchi: J Colloid a d Interface Science, 329, 396 (2009)[9] R. Furuguchi and Y. Toge: Japan Patent Publication2002-155006 (2002)[10] H. Morita, J Ida, O.Ota, K Tsukamoto, and T. Ohmi: Solid State Phenomena Vols., 76-77, 245-250 (2001)[11] H. Morinaga: Jpn Soc Appl Phys, 70, 1067 (2001)[12] Y. Toge: Industrial Cleaning, 4, 13 (2009)[13] T. Takenouchi: J Appl Electrochem, 40, 849 (2009)[14] U. Sato, T. Takenouchi, H. Hara, and S. Wakabayashi: Transactions of the Japan society of mechanicalengineering, 72, 722C, 328 (2006)12

[15] U. Sato, T. Takenouchi, H. Hara, T. Yamazaki, and S. Wakabayashi: Transactions of the Japan society ofmechanical engineering, 72, 718C, 1987 (2006).
[16] U. Sato, T. Takenouchi, H. Hara, T. Yamazaki, and S. Wakabayashi: Transactions of the Japan society ofmechanical engineering, 71, 710C, 3074 (2005)[17] Y. Toge, S. Saida, and R. Furuguchi: Japan Patent Publication2006-150272 (2006)[18] T. Takenouchi: Ziryo-to-Kankyo, 61,495 (2012)[19] T. Takenouchi, U. Sato, and S. Wakabayashi: J. Appl. Surface Finishing, 1, 326 (2006)[20] http://www.fujik.co.jp/-------------------------------*JIPCOM LLC
526-0034, 16Yataka-cho, Nagahama-city, Shiga Pref., Japan
URL: www.jipcom.jp
E-mail: y-nishio@jipcom.ne.jp13