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Corrosion protection wall of Donper stainless steel pressure water

tank(Huangshi Donbei Mech-Electrical Group Solar Energy Co., Ltd, Technology

department, Mr. Shi Xiongyan)

Abstract: This article introduced the Donper patent corrosion protection wall technology, and

research its application and principle on stainless steel water tank, and analysis its reliability

on stainless steel water tank. At last it spells out the result’s actual guiding significance for the

solar heating, gas boil heating, heat pump water heater industry.

Key words: Corrosion protection wall Stainless steel pressure water tank Passivation SUS304

stainless steel

Introduction: With the high speed social development, the requirement for the quality of life is

higher and higher, the demands of hot water is increasing year by year. There are variety hot

water storage methods, but at present, mainly rely on the water tank . The hot water storage

tank are mainly applied to solar heating, heat pump system, gas boiler heating industry, and

the demands is increasing year by year, especially the stainless steel pressure water tank.

But the problems like corrosion and leakage of water tank had always troubled the consumers.

According to the market survey, there are countless water tank occurred leakage because of

corrosion, especially the pressurized pressure water tank. So, the research of corrosion

protection wall of the stainless steel pressurized pressure water tank is very important.

The corrosion protection wall technology is researched by Donper Solar which depends on

different usage of pressure water tank, which resolved the corrosion problem of the pressure

water tank.

1. The principle

1.1 Formation mechanisms of corrosion protection wall

Patent of Donper stainless steel pressure water

tank

Patent no.：ZL 2012 2 0394501.9

Patent no.：ZL 201420041158.9

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The corrosion protection wall mechanisms can be explained by film theory, which means that

because the interaction of metal and oxidizing agents(passivation liquid) will produced a

passivation coating(it is very thin, high density, good coverage performance and can be

adhesive on the metal surface) on the surface of metal. This coating is a independent phase,

it is usually the compound of oxygen and metal. The metal and corrosion medium is fully

isolated by this passivation coating, avoid the directly contact of the metal and corrosion

medium, and made the metal stop dissolution. After oxidizing agents treatment, it will

occurred the passivation coating on the surface of the austenitic stainless steel.

1.2 From self-passivation to passivated

SUS304 stainless steel is austenitic stainless steel material. An important feature is the

self-passivation function, it can produce a oxidation film, which can avoid anodic reaction, to

decrease corrosion rate. But this passivation coating is very thin(its thickness is only

0.02~1.0μm), and not very even(especially the material been processed), it will be pitting

corrosion in some certain medium. Such as the halogen ions will broken the passivation film

on the surface of stainless steel, and made the passivation film loss its protection function.

To improve the anti corrosion function of austenitic stainless steel material, Donper

researched a SUS 304 stainless steel repassivation (corrosion protection wall) technology to

modify the corrosion potential and then improve its corrosion resistance.

1.3 Formation process and passivation process anode polarization curve

The surface of stainless steel corrosion protection wall adsorp by the hydrone and by

oxidizing melting, then formed oxide and hydroxide through oxidizing agent, and produced

conversion reaction with Cr, Ni element, at last formed the coating, avoid other element to

broken the coating and occurred corrosion, its main chemical equation as below:

Fe.H2O+Os=[FeOH. Os]ad+H++e

[FeOH. Os]ad= [FeO. Os] ad+H++e

[FeO. Os] ad+ H2O=FeOOH+ Os+H++e

[FeO. Os] ad= FeO+Os

FeOOH+Cr+ H2O=CrOOH+Fe. H2O

2FeOOH=Fe2O3+ H2O

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2CrOOH= Cr2O3+ H2O

Ni+FeO+H2O=NiO+Fe·H2O

( Os means the catalyst, ad means the surface adsorption intermediate), the surface of

SUS304 stainless steel passivation coating including Fe2O3、Fe(OH)3 or Cr2O3、CrOOH or

Cr(OH)3, the passivation coating includingCrO3、FeO and NiO.

The four areas of its metal passivation process anode polarization curve can be shown

by figure1:

Part A-B-- Active dissolution area

The metal was dissolution as normal anode, the curve is start from metal corrosion potential,

the current is increase with the potential increased. Fe Fe2+ ＋ 2e

Part B-C-- Transition area

When the electrode potential reaches a passivation potential (Epp), it reaches maximum

current (imax), at this time, the state of the metal surface mutates, starts passivation, the

current density declines sharply.

3Fe＋4H2O Fe3O4 ＋8H+ ＋8e

(3) Part C-D--Passivation areaWhen the electrode potential reaches a preserved passivation potential (Ep）), the metal is in a

stable passivation state, the passivation film layer of good corrosion resistance generates on

the surface . Current density (ip) is almost independent of the electrode potential.

2Fe＋3H2O Fe2O3 ＋6H+ ＋6e

(4) D-E area-- Transpassivity areaWhen the electrode potential is further increased beyond the Ept , the current increases with the rise

of the potential, the passivation film is damaged, the corrosion increases.

4OH- O2＋2H2O＋ 4e-

Fe2O3+4H2O Fe2O72-+8H++6e

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Figure 1, Anodic polarization curves in Metal passivation process

Figure 2, Metal passivation behavior in different oxidbillity medium sketch

We can see from Figure 2, oxidizing strength of different oxidant (lineⅠto line Ⅳ) , will cause

different potentials and the current density ,when in the oxidant (line Ⅲ), the metal is in a

stable passive state. Donper Company through researching SUS304 stainless steel

anti-corrosion wall passivation solution, uses oxidant at (lineⅢ), to achieve good results.

1.4 Anti-corrosion Wall (passivation film) features

Three characteristics of stainless steel anti-corrosion wall cause its corrosion resistance:

firstly,the anti-corrosion wall is very thin, in the condition of chromium content> 10.5%, usually

only a few microns; secondly, the proportion of anti-corrosion wall is greater than that of the

base . These two features indicate anti-corrosion walls is thin and dense, it is difficult to be

stoved by corrosion medium to rapidly corrode the substrate. Thirdly, the chromium

concentration of anti-corrosion wall is double of the substrate.. Therefore, anti-corrosion wall

is with high anti-corrosion resistance.

2 Anti-corrosion wall technology applications on stainless steel pressure tank and

principle

SUS304 stainless steel materials have better overall performance, processing performance

and good anti-corrosion resistance, widely used in various industries, thermal storage tank is

no exception. But stainless steel in the actual application of thermal storage tank appears

corrosion, pitting corrosion, crevice corrosion, intergranular corrosion in common. So what

causes these stainless steel corrosion phenomena? To address this problem, we conducted a

study to collect market information, mainly in the following areas:

(A) Severe corrosion occurs within the crevice.Structural design is unreasonable, during the

cathodic reaction, the cathode reactant oxygen is difficult to enter into the structure crevice,

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which leads to the concentration differences of the inner and outer surfaces of the crevice,

there will be concentration difference cell corrosion, that is crevice corrosion .

(2) Surface welded severe corrosion (pitting and intergranular corrosion). After analysis,

mainly due to in the production process of pressure tank, the material in machined,

high-temperature welding process, the oxide film of the surface is destroyed and the

metallurgical structure of the material is changed .In a certain medium, as the Cl- ion

erosion, local corrosion easily initiates at these weak points, the non-uniformity of the surface

film chemical composition impacting passivation film protecting performance is much

stronger.than the physical defects .The anti-corrosion wall technology application of the tank can greatly improve the

anti-corrosion resistance of the product, because the stainless steel pressure tank through

the passivation , the iron oxide is preferential dissolution to chromium oxide,depleted

chromium layer is removed, so that chromium enriches on the stainless steel surface ,this rich

chromium passivation film electric potential is up to + 1.0V (SCE), close to the precious metal

electric potential , improve the anti-corrosion stability.

3 The anti-corrosion wall technology application reliability on the stainless steel tank .

Against the anti-corrosion wall technology application reliability on the stainless steel tank , to

determine the reliability of the technology, Donper company does a lot of experimental

research, including salt spray contrast test , high-temperature saline contrast test, FeCl3

anti-pitting corrosion test, etc., relatively good results received .

Test description:

Test one:

Taking the same two automatic arc welding of SUS304 stainless steel sheets, one after

passivation, another one did not make passivated, samples were placed in 5% of saline and

fresh water, (see Figure 3).

Purewater

not passivationembalmed

concentration of5% saline

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Results: After two months of immersion test observation, embalmed SUS304 stainless

steel at a concentration of 5% saline, basically no rust appears. But not embalmed SUS304

stainless steel in a serious corrosion in water.

Test two:

Taking the same two automatic arc welding of SUS 304 stainless steel sheets, one after

passivation, another one did not make passivated, samples were placed in 5% of saline salt

spray test chamber, salt spray test, which lasted : 16h.

No. Test time Test picture Phenomenon

304 stainless steelarc welding sheetwithout passivation

Before

test

After test

Serious weld

corrosion

Passivated 304

stainless steel arc

welding sheet

Before

test

After test

No weld corrosion

passivationembalmed（Figure 3 Saline soak contrast test）

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（Form 1，Salt fog contrast）

4. Application of "antiseptic wall" technology on the tank actual effect

Application of "antiseptic wall" technology on the tank actual effect

In order to verify whether the company's pressure water tank is corroded, in August 2013 the

company organized a "new for old recall" pressure water tank events, random and "recall" the

use of 8-year water tank, a total of eight units, after dissected by Technology Development

Department, found liner welding are intact, there is no evidence of corrosion. (As shown in

figure 4)

(Figure 4 The topography of Donper pressure water tank which after used for 8 years)

5. Conclusion

This article describes the "antiseptic wall" technology and its application in heat pump water

tank, solar water tank, boiler water tank and other industries, and through verification shows:

Donper "antiseptic wall" stainless steel pressure water tank corrosion treatment after welding

has withstood the test of time, it can be said Donper stainless steel pressure water tank

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"antiseptic wall" technology is excellent.

References:

1 Chinese Society for Corrosion and Protection, localized corrosion of metals, Chemical

Industry Press, Beijing, 1997

2 Jinbao Zhou, stainless steel passivation development process, the Aviation Industry

Corporation, Taiyuan, 1995

3 Xuanyi Wang, stainless steel passivation film progress, Beijing University of Science and

Technology, Beijing

About the author:Xiongyan Shi, universities, engineers titles, solar thermal utilization engineer

(senior), Huangshi Dongbei Mech-Electrical Group Solar Energy Co., Ltd., is responsible for

the research and development of solar pressure system, stainless steel pressure water tank

and solar collectors.