Enamel is often used to provide a good corrosion to resist electric heating/solar heating and warm water of the tank water . Compared to other substrates , the main reason to choose enamel spray paint is it's longer service life and excellent value for money;Currently,enamel liner is one of the relatively advanced liner in the market,with the corresponding advantages in the pressure anti-acid, anti-corrosion areas.
Enamel is a composite material which make inorganic vitreous material by melting then point on base metal and work closely together with metals .Because it needs to make one material fused on another material, chosen for the sheet which is a special enamel steel ,it is low carbon content,and organization metallographic, and surface status and the mechanical performance are meet enamel of special requirements, which yield degrees up 340MPa, expansion coefficient and enamel layer similar even same, such, tank in constantly of hot and cold alternating changes process in the, within bile enamel surface still does not appeared crack and de porcelain phenomenon.Enamel powder is composed of three parts:the mineral raw materials, chemical raw materials and coloring materials.Three part of the raw materials need to be precision ratio by craftsman, grinded balls ,enamel,then by the high temperature of 860 ℃ melting and rapid cooling into borosilicate glass.
In enamel stage,if you wang to use disposable overall enamelling technique,you can choose Shanghai LiangShi's leading spray enamel-lined equipment and roll enamel-lined equipment,during the enamel section , configure the enamel pulp ,enamelling and drying sintering these three processes ,each process in the grasp of the parameters is very strict.

Pressure inner tank automatic powder spray enamel production line	Pressure inner tank automatic powder enamel coating system
Haier water heater pressure inner tank spray enamel automatic production line	Haier water heater roll bearing liner lining system equipment

Water heater pressure inner tank sandblasting and enamelling system equipment successful partnership:
Haier Group Zhejiang ShuaiKang Group JiangSu JiangSu MaiNeng Gomon
NanJiang O.A Smith Water Heater HuaYang solar water heaters
Overseas successful projects:
Export Georgia water heater liner blasting enamelling production line
Export Vietnam water heater liner blasting enamelling production line
Export India water heater liner blasting enamelling production line
Export Singapore water heater liner blasting enamelling production line

Enamel water heater

1.Introduction
Enamel water heater originated in the United States 50 years ago.In Europe, especially Germany, enamel interior water heater production began in the 1960s.But only when the enamel interior combined with the sacrificial anode,it has established it's position in the field of application. Water heater's capacity can be as small as 15 liters, large to 4000 liters, roughly divided into two kinds :american and european, the main difference is the interior design.
In recent years, in the production of water heaters, enamel inner tank is on the rise.Enamel inner tank due to it's advantages in corrosion, is replacing other inner tank and became the main inner tank.

2.Water heater enamel technology
Table 1 is the brief description of the enamel inner tank of the water heater production process.The following will describe the main features of the various steps throughout the process.

3.Quality requirements for tank steel plate
Not all steels are suitable for enamelling.When choosing steel, we should pay attention to several factors:can be coated, surface defects, formability, sag, strength and weldability.The relative importance of each factor, depending on the requirements of the finished product.
According to the water heater enamel inner tank 's requirements ,standard and conditions of actual use,selection of steel plate in enamel inner tank of the water heater you should consider the following conditions:
1. According to the structure of the enamel inner tank and product's compression under standard conditions ,select plate with a certain thickness, the yield strength can meet the above requirements,and there are certain safety assurance.
2. Selected steel grades should match the selection of porcelain enamel.Select the sheet steel used should have a certain fishscale performance, by examining and adaptability painted enamel tests can confirm whether the steel is suitable for the production of ceramic liner.
3. Good surface quality of steel.
At present, the domestic production of steel sheet for enamel inner tank of the water heater, because of differences in product variety and style, uses plate type range is wide.There SPP represented in special steel enamel; to SPCC, SPCD, SPCE (including St12, St13, St14, etc.) as the representative of an ordinary sheet steel; to St37-2 (including France SOLLACG company Emaile245, Emaile265, Emaile310 etc.) representatives of steel, more than three main chemical components and mechanical properties of steel portion of the data is as follows:
Type
Steel grade Chemical composition% Mechanical properties（N/mm2) Notes
C Mn Yield Strength Tensile strength 
SPP <0.008 <0.50 136-205 270-303 
SPCC <0.12 <0.50 172-215 >275 St12
SPCD <0.10 <0.45 >275 St13
SPCE <0.08 <0.40 >275 St14
St37-2 <0.17 <1.4 >235 360-510 
Emaile245 ≤0.14 ≤0.7 250 360-450 
Emaile265 ≤0.17 ≤0.9 280 430-510 
Emaile310 ≤0.17 ≤1.0 310 430-550 

4.The quality requirements for welding
To meet the standards of the water heater manufacturer, enamel interior design is the key. After selecting steel, has a good fit for enamelling welding quality is an important factor to ensure product quality.
Enamel interior design and body weld machining precision deep premise is to ensure the quality of welding, weld product has to be good enamel painted has been an important factor which must be adapted in the process .In a sense, welding quality, determines the quality of products after the enamel coating.Because the welding weld defects caused by the inner wall of the weld zone as the liner had burned, it will have some serious defects such as more large pores,bubbles, large inclusions,cracks and not welded together etc , and in weld and hot effects district appeared serious of widmanstatten ,will leave a variety of enamel defects after firing ,it will effect anti-corrosion performance of products , serious of welding defects will directly effects products of security performance.
The requirements of the weld of products are:structural precision welds, each part of the inner wall of reasonable shape,suitable for enamelling process requirements,painted enamel surface of the weld zone after the enamel firing,the organization was essential normal. With the rapid development of technology, many advanced welding technology and processing technology to the traditional weld width in millimeters greatly reduced,so that the thermal impact of the weld zone is significantly reduced,make product's quality more suitable for the requirements of current enamel coating process.

5.Pretreatment
In order to achieve the aim of enamel corrosion and coating must be close, not hollow, and have strong adhesion of steel at the bottom. Therefore, it is necessary for cleaning and pretreatment of steel.
There are two kinds of pretreatment methods:chemical and physical methods.
Select the physical law firms they use sandblasting process. Sandblasting can not only clean the surface, but also make the surface rough and can increase adhesion of the coating and steel surfaces,select sand blasting method the factors you need to consider are stains on the surface, surface roughness, and the economy.
Acid is usually used for chemical cleaning method, for steel is normally used sulfuric acid cleaning,for the bi-metal such as aluminum, usually using an alkaline solution (NaOH),after washing,steel had to remove residual acid pickling solution with water.
These two methods ,which is better depends entirely on economy of operation, and other technical factors,but the two methods both can produce a surface suitable for enamelling.

6.Enamelling
Conventional water heater's enamelling usually has two types:wet and dry.
Wet enamelling includes spraying ,dipping and rolling enamelling and other methods.By rotating the liner in the three-dimensional space, so that all internal surfaces are coated with a glaze slurry. Excess residues and liquids must be removed to ensure that the steel surface is covered with a thin layer of glaze, uniform and continuous.Many factors influence the quality of enamels such as the viscosity of the glaze, the pretreatment of steel, adhesion of the metal surface, welding quality, and inner structure.Wet enamelling more popular in the United States.
Dry enamelling is dominated by electrostatic spraying.Compared with the wet system ,electrostatic spraying has many advantages. It reduces costs, saves energy, and material utilization can be reached 97%. In addition, it also reduces pollution of the environment.Currently, some large water heater manufacturers in Europe basically use electrostatic spraying method.
Most water heater manufacturers only spray a layer of enamel.Only the use of a certain temperature and pressure conditions are very strict, such as sinks, using two layers of painting. While others are used in a special pre-coat covered with enamel.

7.Enamel firing
Enamel at 700 ℃, begin to melt and form a ceramic layer.Firing temperature is generally maintained at between 830-860 ℃, time is 7-15 minutes .Red-hot liner was removed from the furnace and gradually cooled to room temperature.Usually firing process is continuous and can repeat it again, to form a second layer of enamel.Even repeated several times to get the demanding conditions required for the application of multilayer enamel.

8.Water heater common pitfalls
Enamel is not without flaw, otherwise there is no need for cathodic corrosion protection.Image 4 shows some enamel defects, such as scales blasting, stripping porcelain, internal inclusions and pinholes.We need to pay attention to the enamel "defects" and "hidden defect" there is a difference.In accordance with the relevant standards, these two defects are defined as follows:
Defects:The enamel defects which is depth of the metal substrate(such as pinholes, cracks, crack and peel porcelain).Defects are usually conductive, protective current consumption.
Hidden defects:Hidden defects are enameled metal oxides-rich spots. It's exterior is dark brown and black dots. Hidden defects may conduct electricity, can consume protection current.

9.Water heater's testing standards and requirements
Project Requirements Standard
Steel quality side scale-free blasting DIN1623/3 Cold-rolled steel
DIN17100，EN10209 Other plates
Ceramic layer thickness 0.15mm—0.50mm,If using two layers of enamel ground coat <0.2mm, overglaze <0.15mm DIN4735/3，ISO2178
Defects, such as burnt edges, points, holes, cracks and other According to DIN4753/3, or according to agreement DIN4753/3，
DEZ-MB7.19,or according to agreement
Adhesion steel<2mm,2 grade
Steel>2mm, Good DEZ-MB7.10.3,EN10209, DEZ-MB7.10.4
Impact resistance
After 24 hours, No>1.5mm diameter injury DIN51155
Hot nasty transsexual 200C°,5 Period DEZ-MB7.12.1
Chemical stability
Tap water according to agreement DEZ-MB7.3.3
Boiling water weightlessness <3.5g/m2 ISO2744,DIN4753/3
Water vapor according to agreement ISO2744
Cold citrate acid A Grade ISO2722
Boiling citric acid according to agreement ISO2742
Cold hydrochloric acid A Grade(10%HCl,1 Hour) ISO2722,DIN4753/3
Focal linsuanna <20g/m2 DEZ-MB7.4.4
Physiological requirements Limit DIN51032 DIN51031

11.Quality Inspection
11．1 The thickness of the enamel
Enamel thickness is related to the adopted technology . no matter single layer or double enamel process,the thickness of the enamel should be between 0.15-0.5mm. Porcelain layer should be uniform and continuous. Usually use a small hand-held instruments to measure the thickness of the enamel. Thickness measurement should be averaged over ten data, measurement data should include bladder and bile cover. If you do not meet the requirements, then assessed as unfit.
11．2 Adhesion
This is a very important indicator to measuring the quality of enamels. In case of destructive testing, requiring no exposed metal surfaces, which must be still covered with enamel on metal surfaces. Good adhesion of enamel technology strength up to 100N/mm2 and common inspection approach is hammering (EN10209).
11．3 Impact resistance
10N force imposed on the enamel specimens, specimen surface enamel damage not exceeding F1.5mm (DIN51155).
11．4 Heat-resistant CML
Method of test for heat-resistant CML is the enamel repeat 5 times,drop from 200 ℃ to 15 ℃ and observe whether there is visible surface damage. Tests require for 3 pieces specimens which area larger than 100cm² .
11．5 Pressure test
After the water heater is full of hot water,pressure,pressure of at least 1.3xP (p is the operating pressure). From 0 to P-charged, not less than 15 seconds, the pressure P to the test, no more than 5 seconds, the test pressure is maintained for at least 15 minutes. The whole process was repeated twice. If the water heater can withstand 12 bar pressure,as qualified.
11．6 Acceptable total area of enamel defects
Hidden defects:Only allowed to exist at the edges and weld, and the distance from the site of the recessive defective weld edges and no larger than 2mm, the total area of ??the enamel surface defects per square meter recessive no larger than 20 square centimeters of.
Defects:Per square metre of surface defects of enamel total area shall not be greater than 7 cm.
11．7 Chemical properties
11．7．1 Resistance to boiling water
According to DIN-ISO2744, test methods are as follows:
Deionized water:conductivity 1ms/m
Water temperature:micro-boiling
Sample:3 pieces, thickness of the porcelain >0.25mm,thickness of steel sheet <2mm
Test time:6 weeks
Steps:
Monday to Friday:Change the water once a day
Saturday to Sunday:Do not change water
After three weeks:Remove the sample, clean, dry, and weighed.
Repeat the process three weeks time.
After the test, enamel cannot be greater than the loss in mass 3.5G/m2
11．7．2 Resistance to acid
With citric acid solution (ISO2722) or hydrochloric acid (DIN4753) in aqueous solution, drop in on enamel samples at room temperature after 1 hour and check for corrosion of the surface, according to the classification criteria for rating. Usually FERRO enamel to reach level a (good) level--AA (excellent).
11．7．3 Physiological harmlessness 
As the water drinkable, therefore, enamel can not contain harmful substances. States have different standards of drinking water, precipitation from the enamel to the harmful substances in the water has certain requirements such as lead, cadmium, arsenic, antimony and so on.

12．Quality control---standard protection current
Although the enamel corrosion protection, but inevitably the enamel there is some discontinuity, or defects. In order to improve the life of the water heater, and 50 in the United States have been using magnesium anodes to protect water heaters. This technology is now mature, so that common standard protection current water heater manufacturer to evaluate the quality of their products.
12．1Test methods and evaluation
Experimental magnesium anode rod diameter is 20-33mm. Magnesium anode rod with insulation installation. Test, add-1.5V power supply. On the positive voltage between the magnesium anode rod, inner negatively. 2 minutes later, measuring current and voltage drop in the line, with the following
formula to calculate current (mA/m2) of:
IM U0 ms
iN= . 
AK UM.f(G).f(K) mR

In the formula: IM the measuring current
f(G) 、f(K)The correction factor
U0 reference voltage,1.5V
UM measuring voltage
ms standard anode weight,（200g/m2）
mR test anode weight
In the relevant standards, it has been standard on the maximum value makes provision for the protection current Guarantee:
single enamel double enamel 
Maximum protection current 22.5mA/m2 15.5mA/m2 

12．2 Precautions:
Since a voltage source applied for the test, so the measurement is in the range of the cathode is carried out adequately protected. Protection current obtained in this experiment is a relative value, not reflect the actual size of enamel defects and recessive defects. Therefore, can not be used to protect current calculation instead of the actual protection current, of course, we can not deny that the relevant standards to protect current and the actual enamel defects and latent defects. Currently, many water heater manufacturers have this method to detect the quality and stability of the enamel process. Commonly used sampling methods, such as a group of checks, one hundred checks a liner.