Enamel Coating on Steel
Enamel Coating on Steel
The art and science of fusing glasses (now known as porcelain enamels or simply enamels) to the surfaces of metals dates back to the civilizations of ancient Egyptians and Persians. The earliest use was for jewelry where the enamel was fused to gold. By the middle-ages, the range of metals capable of being glass-coated progressed from the noble to the base metals such as gold followed by silver, bronze, copper and, in the early 1800s, cast iron. Enamel was first applied to sheet iron and steel in Austria and Germany shortly after 1850. In the 1900s, the enamel industry grew rapidly, with many new applications such as appliances, hot water heaters and architectural panels.
Present-day enamelling processes have been developed over the course of the 20th century, mirroring progress in steel production, but also keeping pace with ever-stricter environmental norms. Today, enamelling steel has become a high-tech process using highly sophisticated materials and state-of-the-art techniques. Enamelled steel is a material which meets modern-day requirements of longevity, aesthetic qualities, hygiene and respect for the environment. This ancient art has also now found new application in the industrial electronics.
Enamelled steel has many properties which make it a first-rate material for numerous applications. Its characteristics result from combining the properties of its two constituent elements namely steel and enamel. Steel contributes mechanical strength and formability, while enamel provides durability and a beautiful glossy appearance.
Enamel is a substantially vitreous or glassy inorganic coating bonded to the steel substrate by thermal fusion. This coating is applied for the protection of steel products from surrounding environments. This coating provides not only an aesthetic exterior but also provides outstanding engineering properties, such as mechanical strength of the enamelled surface, multiplicity and stability of colour, corrosion resistance, resistance to wear and abrasion, chemical and heat resistance, resistance to thermal shock and fire, hygiene and ease of cleaning etc.
Enamel is essentially a glass with a low softening temperature ranging from 510 deg C to 530 deg C. It is a glass obtained by fusion at high temperature between 1000 deg C and 1300 deg C. Enamels normally consist of an acidic refractory material such as quartz, feldspar, clays and mica. In order to confer on enamelled parts its properties of durability, silica (SiO2) glass has to be modified, as it cannot be used in its original state. Its melting point is too high, its coefficient of thermal expansion is too low compared to that of steel and its adhesion to steel is zero. Hence, various additives need to be added in order to obtain enamel. Depending on the ultimate function of the enamel, various additives which can be used are pigments, opacifiers, clays or other materials to serve as defloculants and floatation agents, which help suspend the enamel particles in an aqueous solution.
Constituents of enamels
The different constituents of the enamel can be categorized in five main groups, according to the properties they impart to the enamel. These are (i) refractories, (ii) fluxes, (iii) adhesion agents, (iv) opacifiers, and (v) colouring agents.
Refractories provide enamel an amorphous structure, and hence mechanical strength. These include, as an example, alumina (Al2O3), which serves to lower the coefficient of expansion, increase resistance to temperature, chemicals and abrasion, and facilitate the action of opacifiers.
Fluxes lower the melting point and firing temperature and increase the coefficient of expansion. Refractories are reacted with the fluxes to form glass. The fluxes are mainly composed of borax (sodium tetra-borate in its anhydrous form (Na2B4O7) or hydrated form (Na2B4O7, 10 H2O)) and alkaline oxides such as oxides of sodium (Na2O), potassium (K2O), lithium (Li2O), calcium (CaO), magnesium (MgO) and strontium (SrO). These constituents produce boro-silicates of sodium, potassium, lithium, calcium, magnesium or strontium, which have a lower melting point than SiO2 (around 1400 deg C instead of 1720 deg C). The melting point can also be lowered by adding fluorine (F2) or boron trioxide (B2O3). Fluxes, such as the alkaline oxides, increase the coefficient of expansion by filling the ‘voids’ in the silica structure.
Adhesion agents are metal oxides which are involved in chemical redox reactions for promoting adhesion between the steel surface and the enamel coating. These reactions also involve the iron (Fe) and carbon (C) in steel, as well as atmospheric oxygen (O2). Adhesion agents are present in ground-coat enamel, mainly in the form of molybdenum oxide (MoO), cobalt oxide (CoO), cupric oxide (CuO), manganese oxide (MnO2) and chromic oxide (Cr2O3). Nickel oxide (NiO) is definitely the most efficient adhesion agent. However, enamel producers have banned its use for reasons linked with food contact safety and REACH compliance.
Opacifiers and colouring agents contribute visual and tactile qualities to enamelled parts. Opacifiers serve to increase the opacity of enamel and are present in cover-coat enamel. The most common opacifiers are titanium dioxide (TiO2), antimony oxide (Sb2O5), zirconium oxide (ZrO2) and tin oxide (SnO).
Colouring agents are obtained by combining mineral oxides. The colour of enamel depends on the type of colouring agent, its concentration in the enamel, the chemical composition of the enamel and the firing conditions in the enamelling furnace. The most common procedure for the colouring agent is that it is to be mechanically mixed in the form of fine particles with the enamel during the grinding stage, before application to the steel substrate.
Types of enamels
There are different types of enamels. Enamels have different compositions depending on the type of substrate to be coated and depending on the enamelling process used. The composition of enamels is varied in order to match the enamel firing temperature to the substrate. The higher the firing temperature, the better is the quality of the enamelled parts. As far as the steel is concerned, however, the firing temperature is limited by the substrate. As an example, phase changes in steel are to be taken into account. Also, the coefficient of expansion of enamel is to be compatible with that of the substrate.The different types of enamel for steel are (i) ground-coat enamel, (ii) self-cleaning enamel, and (iii) cover-coat enamel.
Ground-coat enamel contains metal oxides (Co, Cu oxides), which promote enamel to steel adhesion by creating alloys with the Fe in steel. Since metal oxides are dark in colour, white ground coat does not exist. There are more reactive ground-coat enamels which contain a higher proportion of metal oxides. This avoids the need of pickling the steel before enamelling. These enamels are used, as an example, for the two-coat/one-fire enamelling process. Ground-coat enamel also protects against corrosion of the enamelled part. Also, depending on the type of part to be enamelled, other constituents are required to be added for getting (i) acid-resistant properties (oven cavities, dripping pans) by the addition of TiO2, (ii) alkali-resistant properties (sanitary ware, washing machines) by addition of ZrO2, and iii) improved corrosion resistance for water-heater applications by addition of ZrO2 and Al2O3.
Self-cleaning enamels are used in domestic ovens and help eliminate the fat produced when food is cooked. There are two types of self-cleaning enamels namely (i) catalytic, and (ii) pyrolytic. Catalytic cleaning takes place while the oven is working (usually at around 200 deg C). Enamel contains oxides which catalyze the breakdown of fat, forming water and CO2 (carbon dioxide). Further, this type of enamel is very refractory, hence porous, which has the effect of increasing the contact surface between the enamel and the fat, thus facilitating its elimination by the reaction CxHyOz + catalyst = yH2O (g) + xCO2 (g). The efficiency of this reaction falls off over time as a result of progressive blockage of the pores. Pyrolytic cleaning takes place while the oven is empty, at around 520 deg C. Fat and residues which are deposited on the walls during cooking are burnt at this temperature, leaving only a C deposit which can be wiped off. The reaction which takes place is CxHyOz + heat = xC + yH2O (g). This type of enamel has a softening point higher than the pyrolytic temperature. It is glossy, non-porous and highly resistant to acids and alkalis.
Cover-coat enamels give enamelled parts their aesthetic quality and also help to increase their chemical resistance. Since these enamels contain absolutely no adhesion agent, they cannot be used alone on a metal substrate under any circumstances.
Production of enamel
There are several stages in the production of enamel. The first stage is to check, weigh and mix the various constituents of enamel which can be upto 15. This is followed by the fusion stage. The purpose of the fusion process is to render the final amorphous structure of the enamel uniform and to lower the firing temperature. This requires the ‘glass’ to be melted at a temperature of between 1100 deg C and 1300 deg C, depending on the desired composition of the enamel. The most common process involves using a tunnel furnace (gas or electric), where the mixture is introduced at one end and comes out at the other. Movement through the furnace is by gravity. The mixture remains in the furnace about an hour.
When it emerges it is cooled rapidly, first of all by being passed through a water-cooled rolling mill to form a glass sheet, then into a cooler. Crushing completes the cooling cycle. This rapid quenching process, from a high temperature, fixes the structure of the glass and prevents any phase separation. An alternative fusion process involves using a rotary furnace into which the constituents are poured, mixed and heated. Liquid enamel is then poured into a pit and water quenched. After cooling and crushing, enamel frit is obtained.
Enamel frit cannot be used as such. It is to be first mixed with other ingredients, then ground. Enamel can also be applied in liquid form. In this case, after the addition of certain ingredients (suspension, refractory, colouring agents, electrolytes and opacifiers), the enamel frit is ground and mixed with water to form a slurry. This slurry is then used for dip coating or spraying applications. Enamelling persons, who do long production runs in a single colour, are increasingly reluctant to prepare the enamel themselves. This has led to the introduction of ‘ready-touse’ powder in the 1980s to simplify the preparation of the slurry. The powder is prepared by the enamel producers by adding specific products before grinding. The enamelling persons have only to add colouring agents (optional) before mixing the powder with water to get the slurry.
Enamel can also be applied in the form of powder, obtained by grinding the enamel frit. The grinding time is determined experimentally. The ground powder then has to be sieved for getting rid of lumps and various residues, and then passed through a magnetic separator (permanent magnet or electromagnet) to eliminate any iron particles in the powder. The iron particles tend to create ‘holes’ in the enamel, which reduces the corrosion protection of the steel. Finally, the grains of enamel are coated with silicon, enabling them to adhere to the steel substrate between the applications and firing stages. The enamel powder obtained does not require any additives or further treatment at the enameller end and can be directly used in powder spray guns. It takes quite a good investment to set up a powder unit, as it has to be electrostatic to be financially viable. However, this process is more economical in the long term.
The enamelling process
The enamelling process involves applying and firing one or more layers of enamel on one or both sides of a suitable steel substrate. Successful enamelling is characterized by (i) good adhesion of enamel to the steel, and (ii) a good surface appearance after firing of the enamel. The C content of the steel can hinder the process of achieving these two properties. C content of steel is important for ensuring the adhesion of enamel. However, if the C content is too high then it can adversely affect the surface appearance of the enamel because of the release of gaseous CO2 and CO (carbon mono-oxide) produced during firing. This contradiction explains the variety of enamelling processes which exist.
The enamelling process normally comprises several steps (Fig 1) namely (i) preparation of the surface of the part after forming, (ii) preparation of the enamel, (iii) application of the enamel to the steel, (iv) drying, and (v) firing at high temperature. There are several enamelling processes, depending on the type of part and final appearance required. These processes are (i) enamelling on hot rolled steel substrate, (ii) conventional enamelling (two coats/two firings), (iii) ground-coat enamelling (one coat/one firing), (iv) direct-on white enamelling, and (v) two-coat/one-fire enamelling.
Fig 1 Steps in the process of enamelling
Surface preparation before enamelling – The purpose of surface treatment is to get a surface which is compatible with the enamelling process. Surface treatment consists of several steps. The number of steps differs according to the enamelling process used. The various surface treatment steps are (i) shot blasting, (ii) degreasing, (iii) rinsing, (iv) pickling, (v) acid rinsing, (vi) nickel deposition, (vii) rinsing, (viii) neutralization, and (ix) drying.
The purpose of shot blasting treatment of hot rolled steel substrate is to increase the surface roughness of the steel. This provides a better keying of the enamel, as during the enamel firing cycle, enamel-metal chemical reactions are promoted by the resulting higher contact surface area, thus increasing enamel to steel adhesion. Together with the chemical hydrogen (H2) traps created during steel production, this improves resistance to the defect known as ‘fish scale’. Shot blasting is carried out on an unoiled steel substrate for avoiding contamination of the shot. If the shot does become contaminated with oil, it is less effective and soils the surface of the steel. This soiling can cause the enamel to be rejected where a wet application process is to be used.
The purpose of degreasing is to remove exogenous matter such as rust preventing oils, drawing oils and various kinds of dust present on the steel surface and originating from earlier steps. It is therefore a very important step in the process of surface preparation. Different parameters which need to be checked are (i) type of degreasing agent, (ii) degreasing temperature, which is to be in the range of 60 deg C and 90 deg C, depending on the process used, (iii) concentration of degreasing agent normally in the range of 45 grams per litre (g/l) to 50 g/l, (iv) pH of the degreasing baths, (v) treatment time consisting of 15 minutes of immersion and a few minutes of spraying, and (vi) possible mechanical action (agitation of the bath or spraying pressure).
Alkaline degreasing process is the maximum used process. Alkaline solutions can have three different physico-chemical actions namely (i) saponification action where fatty substances are dissolved in the presence of an aqueous solution of soda or potash, forming soluble soaps, (ii) emulsification action where fats are dispersed in fine droplets by phosphates or silicates, and (iii) action of decrease in surface tension where fat molecules are coated with organic agents, which weakens the bonds between them and the steel substrate.
Two methods are employed which are (i) immersion (or dipping), and (ii) spraying. In the case of degreasing by immersion, several baths are arranged in series. The mechanical action (agitation of the bath, spraying pressure) increases the effectiveness of degreasing. Inadequate degreasing causes surface blemishes, demonstrating the importance of monitoring the conditions under which this process is carried out. Another thing which is needed to be watched out is the phenomenon of resinification of the oil on the part to be degreased, which when exposed to light makes degreasing very difficult, even impossible. Lastly, surface blemishes in the steel (scratches, pores etc.) can trap oil residues which can form gases during the enamel firing cycle.
Rinsing follows degreasing and is carried out in one or more steps such as (i) a single hot water rinse (at temperatures of 60 deg C to 70 deg C) if there are to be subsequent surface treatment processes (pickling etc.), (ii) hot rinsing, cold rinsing and a final rinse in DM (demineralized) water if there are no further surface treatment processes.
Pickling of steel is carried out with the purpose to attack the steel surface so as to increase its micro-roughness and hence its reactivity. This promotes the adhesion of enamel. Pickling is generally carried out using concentrated sulphuric acid (H2SO4) and the effect is mainly centred on the grain boundaries. The intensity of the pickling is measured by iron (Fe) loss. Depending on the process used for enamelling, pickling can be light with the Fe loss of around 5 grams per square metre (g/sqm) per side or strong with Fe loss of around 25 g/sqm per side. The chemical reaction which takes place during pickling is iron + sulphuric acid = iron in solution in the acid + hydrogen gas (Fe + H2SO4 = FeSO4 + H2).
During the process of pickling, standard conditions consist of (i) temperature of the acid bath at around 75 deg C, (ii) H2SO4 concentration at around 7 %, (iii) pickling time in the range of 10 minutes to 15 minutes, and (iv) concentration of Fe in the pickling bath at around 2 g/l. The slightest variation from these standard conditions can cause a significant variation in Fe loss, and hence in enamel adhesion.
Chemical analysis of the steel is also an extremely important parameter for checking iron loss. Some elements such as phosphorus, copper or molybdenum have a major influence on its value. It is hence vital to accurately check the concentration of the different chemical elements in steel. In the case of direct-on white enamelling, pickling is the key step for obtaining good-quality parts. After pickling, the surface pattern observed varies according to the Fe loss measured.
In the case of direct-on white enamelling, it is essential that nickel is deposited on the part after pickling for ensuring good adhesion of the white enamel to the steel. The quantity of nickel which can be deposited on a part at a given temperature, the nickel deposition time and the nickel concentration in the bath varies according to the pH. The maximum quantity is achieved at the pH level of 2.8. However, it has been found that after the pickling process, the pH at the surface of the part is less than 1. If rinsing is not carried out after pickling, the pH level remains too low and insufficient nickel is deposited. If the part is rinsed in water, the result remains the same. Hence, the purpose of acid rinsing is to increase the pH of the part, without exceeding the optimum value.
Nickel plays an important role in the direct-on white enamelling process in promoting enamel adhesion. If necessary, it can be used in small quantities in conventional processes (nickel flash).The popular method used to deposit nickel is the precipitation of metallic nickel by displacement of Fe ions as represented by the equation 2Fe + NiSO4 + H2SO4 = 2FeSO4 + Ni + H2. During the process of nickel deposition, standard conditions consist of (i) NiSO4 in the range of 12 g/l to 15 g/l, (ii) pH level at 2.8, (iii) temperature at 70 deg C, and (iv) time at 7 minutes. The conditions of nickel deposition have a significant influence on the quantity of nickel deposited. A slight variation can have serious effects on enamel adhesion.
For ensuring that the direct-on white enamelling process produces enamel with good adhesion and an attractive appearance, it is essential that an optimum combination exists between the Fe loss to be obtained (in the range of 25 g/sqm to 50 g/sqm) and the quantity of nickel deposited (nickel coating in the range of 1 g/sqm to 2 g/sqm per side).
The purpose of the final rinse is to eliminate all traces of acid still present on the surface of the part. Two baths are generally used for final rinsing. The first bath has parameters of pH value in the range of 2.5 to 3.2 and the temperature in the range of 30 deg C to 35 deg C. The rinsing is done for 7 minutes. The second bath has parameters of pH value in the range of 3.5 to 4 and the temperature of around 25 deg C. The rinsing is also done in the second bath for 7 minutes.
The purpose of neutralization is to completely eliminate any acid residues. The neutralization bath has parameters of pH value in the range of 10.5 to 11.5 and the temperature of around 70 deg C. The neutralization is also done for 7 minutes.
Once surface preparation is complete, the parts are to be dried to prevent them rusting before enamelling.
Enamel application – Enamel can be applied using either the wet or dry process. There are many methods of applying enamel by means of the wet process. These methods are described below.
In the case of dip coating, the parts to be coated are plunged into an enamel slurry (mixture of enamel powder and water), the density and viscosity of which are closely monitored. The parts are then suspended to allow the excess applied enamel to drip off, thus ensuring a uniform coating thickness. This process is frequently used for parts which have a complex shape, such as oven cavities. One drawback of this process is that sagging of the enamel can occur. A variation of dip coating, the ‘dip and shake’ method, which involves moving the parts about different axes when they emerge from the bath, thus minimizing sagging and excessive thickness of the enamel coating.
In case of flow coating process, the process entails spraying the entire surface of the part with enamel through one or more round nozzles.
In case of air-assisted spraying, enamel is sprayed on the parts to be coated using a spray gun powered by a jet of compressed air at a pressure ranging from 3 kg/sq cm to 4.5 kg/ sq cm. The process is usually carried out in a booth, the parts being hung on a metal conveyor belt. Manual spraying requires highly experienced operators in order to avoid sagging and excessive thickness of the enamel coating. This process can be automated and tends to be reserved for short production runs.
In case of electrostatic spraying process, a charge differential is applied between the negatively charged enamel and the positively charged parts to be coated. The enamelling spray gun consists of a central tube through which the enamel passes, surrounded by an annular nozzle through which the atomizing air passes faster than the stream of enamel. This difference in speed causes the enamel slurry to atomize into fine droplets. At the tip of the spray gun, the droplets pass through an atmosphere which has been ionized in an electric field and become negatively charged before being deposited on the part to be enamelled. Once the initial coats have been applied, the droplets are less and less attracted to the part and a repulsive force arises. This opposes the attractive force until equilibrium is reached, thus controlling the thickness of the coating. The resulting enamel coating is uniform and losses are minimized.
The electrophoresis process also known as electrostatic dip enamelling or ETE (Elektro-Tauch-Emaillierung) process is principally used for direct-on white enamelling. Particles of enamel, in colloidal suspension in a saline solution, are conveyed under the effect of an electric field. These particles become negatively charged on the surface and are carried to the anode of the electrolytic cell, which is the part to be enamelled. This process results in a very uniform thickness of enamel (automatic limitation of deposition) and an exceptional surface appearance. It is a very efficient technique for flat parts. The drawbacks of the process are (i) it is an expensive process, (ii) it needs the use of a cathode having the shape of the part to be enamelled, and (iii) monitoring the electrical properties of the enamel slurry is quite complex.
Enamel application by the dry process is done by electrostatic powder spraying. The principle of electrostatic powder spraying is the same as for the wet method. An electric field is formed between the nozzle electrode and the part to be enamelled. The particles of enamel, propelled out of the spray gun by a stream of air, become negatively charged, migrate towards the part to be enamelled (positive electrode) and are deposited there. Once the first coat has been deposited, the particles start losing their attractive force. A repulsive force is then generated. When this becomes equal to the attractive force, the particles are no longer deposited. This process thus provides a uniform enamel coating and automatically limits its thickness.
The particles of enamel are to be coated (organic envelope, generally silicon) in order to prevent hydration, which can have the effect of reducing their electric resistance, thus preventing correct deposition of enamel on the part. The quality of the organic coating, the grain size and rheology of the powder are key factors for obtaining uniform deposition of enamel and an attractive surface appearance after firing. This process is highly effective for flat parts, but it is more difficult to enamel hollow parts – e.g. oven cavities – because of the Faraday cage effect. The process offers several advantages such as (i) waste reduction, (ii) material savings, and (iii) uniformity of coating thickness.
Drying and firing of enamel – Drying of enamel is a vital step after the wet application of enamel. Moisture, which represents 40 % to 50 % of the mass deposited, can in fact cause localized withdrawal of the enamel during firing. The dry coating obtained is called ‘biscuit’. Air drying is not desirable since the parts can become contaminated by dust particles in the air and residual moisture can remain in the enamel, favouring the formation of ‘fish scale’ defects. Dryers or ovens are to be used at a temperature range of 70 deg C and 120 deg C. Infrared radiation or convection drying is the safest way of preparing parts for firing.
Firing of enamel is generally carried out at a temperature range of 780 deg C to 850 deg C, which is well above the softening temperature of enamel (500 deg C to 600 deg C). It can be done in a box furnace or in a tunnel continuous furnace. Firing time and temperature depend on the thickness of the steel and the type of enamel. Firing is carried out in an oxidizing atmosphere.
Box furnaces are normally used for short production runs and small parts. Tunnel furnaces are rectilinear, U-shaped, or L-shaped and are suitable for long production runs. They are divided into three zones consisting of pre-heating, firing and cooling zones. This permits a controlled increase and decrease in the temperature. The parts, arranged on cradles, pass through these sections, suspended from a conveyor. Air seals, located at the entrance and exit of the furnace, prevent heat loss. The furnaces are mainly electric or gas-fired radiant tube. The heating elements are arranged on the walls and bottom of the furnace. Thermal energy is transmitted to the parts by radiation and convection.
Enamel to steel adhesion mechanisms
The adhesion of enamel to the uncoated steel is achieved by means of chemical reactions which take place during the firing and cooling cycles. The process can be divided into four stages.
In the first stage which is upto 550 deg C, the moisture and the O2 present in the air penetrate the porous enamel and oxidize the Fe in the steel. This causes the formation of a layer of iron oxide at the enamel/steel interface. The atomic H2 arising from the decomposition of H2O diffuses into the steel, recombines as molecular H2 and fills the holes in the steel. The solubility of H2 in steel is increased with the temperature.
In the second stage which is in the temperature range of 550 deg C to 830 deg C, the enamel softens then fuses, forming a semi-permeable layer. This reduces gaseous exchanges with the furnace atmosphere. The iron oxide present at the enamel/steel interface is dissolved by the enamel.
The third stage is around 830 deg C. At this temperature chemical redox reactions take place between the iron oxide layer at the enamel/steel interface, the metal oxides in the enamel and the C in the steel. Fe-Co alloys precipitate at the enamel/steel interface. These are at the heart of the adhesion of enamel to steel. Adhesion is promoted by the roughness of the steel. The dissolved O2 recombines with the C in the steel, releasing gaseous CO/CO2. The intensity of these releases is required to be monitored. The quantity of H2 in the steel is at maximum levels.
The fourth stage consists of cooling. The enamel solidifies, stopping gaseous exchanges. H2 solubility in the steel decreases when the temperature falls. The steel becomes oversaturated and H2 accumulates under the enamel coating. An excessive quantity of H2 at the interface causes ‘fish scale’ defects.
Defects of enamel coating
Defects which cause rejection regarding quality standards for production of enamel coated materials are termed as enamel defects. These defects can be locally limited interruptions of the compactness or structure of the glass like coating. When enamel coatings are discussed, repair or recycle processes are quite difficult to proceed for defective enamel coatings. While the formation of defects can often be attributed to a combination of several unfavourable parameters, which leads to practically infinite number of individual faults, quite often one factor dominates a typical defect type. Hence, the defects are classified in groups given below according to the underlying base materials and application processes.
Fish scales – These are steel-related defects which are half-moon shaped cracks in ground or cover coats, which occur immediately or even hours or days after the firing operation. These defects can occur individually with a typical size of 1 mm to 5 mm in diameters. The defects are the result of H2 diffusion through the steel and into the enamel layer. The defects only occur on pieces enamelled on both sides. The H2 is formed at the steel surface during firing according to the reaction Fe + H2O = FeO + H2. H2 is dissolved in atomic form and after cooling remains in the steel as supersaturated solution. The separation of H2 from the steel takes place by recombination to molecules at the steel/enamel phase boundary, then building in pressures of upto 200 kg/sq cm, which causes scaling.
Poor adhesion – Adhesion of the enamel coating is explained with two basic adhesion mechanism namely (i) chemical theory, and (ii) mechanical theory. Chemical theory indicates that a continuous shift of the type of bond is to be achieved in the region of the phase boundary from the metallic bond of the base metal via an oxide adherence layer to the ionic bond of the enamel layer. Mechanical theory is defined by the prerequisite for good adherence is roughening of the interface surface leading to a tight mechanical clinging of the enamel to the steel surface. The adherence of the enamel coat can be ascertained by destroying it by means of mechanical deforming. Poor adherence of the enamel is a very severe quality issue spoiling appearance which can lead to rapid destruction of the steel/enamel composite. Poor adherence can have very different origins, ranging from non-suited steel grades over poor pre-treatment, application of enamels with too low cobalt/nickel oxide content to under- or over-firing.
Blisters – Blisters are hollow holes through the fired enamel, having a diameter of upto 1 mm, which can remain intact in the enamel surface, but can also blow off leaving a funnel shaped recess. The common cause of this defect, also called re-boiling or C boil, is a local strong gas development during firing, with the gas containing H2 as well as CO. Pickling residues, through their (gaseous) decomposition products can bring up very heavy boiling-up with enamel and steel. Due to the diffusion of the H2 thus arising, impurities can also be observed on the opposite side of the sheet steel. Often, blisters are observed with hollow ware where in sealed rings or badly shaped handles obstinate residues of pickling acid accumulate.
Impurities – Impurities in base coat enamelling can range from sheet steel contamination to scale deposits. Often, it is very difficult and time consuming to find the origin because impurities can be introduced in all steps of the enamelling process. The most frequent ones are (i) fine iron particles from cutting and welding, (ii) residues from pre-treatment agent, (iii) coarse particles from milling and balls (white spots), (iv) coarse (ungrounded) mill additions, (v) dust from cover coat enamel, and (vi) scale deposit from firing tools.
Burn-offs – Burn-offs are localized areas of iron oxide eruptions through the enamel coating. The main causes for these defects are a too thin enamel thickness or an insufficient amount of refractory mill additions. In the first case, the excess iron oxide which is not solubilized in the base enamel penetrates from the phase boundary to the surface.
Properties of enamelled steels
Enamelled steel has several useful properties which are mainly due to the vitreous nature of the enamel. The chemical composition of enamel differs according to its end use for fully meeting the required characteristics. The properties are given below.
Enamel adhesion – The enamel is to adhere to the steel substrate for ensuring that the enamelled steel has the required properties for every end use. Adhesion is determined by means of an impact test, which involves deforming a sample of enamelled sheet using a hemispherical punch by dropping a 1.5 kg weight onto the punch from a height appropriate for the thickness of the steel substrate. The degree of adhesion is determined by comparison with reference photos. The value assigned ranges from 1 (very good adhesion) to 5 (very poor adhesion).
Corrosion resistance – Enamel is a coating which provides steel with outstanding corrosion resistance, even at high temperatures. Enamelled surfaces are non-porous and hence impermeable to all liquids. Salt spray tests performed on enamelled parts with a cold rolled or aluminized steel substrate show that enamelled parts can withstand salt spray for over 500 hours without showing any signs of red rust. Tests carried out by the Porcelain Enamel Institute have shown that enamelled panels can go for 30 years without any signs of corrosion on the steel substrate.
Chemical resistance of enamel – The chemical properties of enamel can be custom-made to the environment in which it is to be used. Enamel thus has extremely good resistance to chemicals (acids, alkalis, detergents and organic solutions). Enamelled steel is also extremely resistant to atmospheric attack. Hence, rain, atmospheric pollution (sulphur dioxide, nitric oxide), salt-laden coastal atmospheres, ultra-violet (UV) radiation and sudden changes in temperature do not lead to any changes in the appearance, colour or gloss of the enamelled surface.
Mechanical strength of the surface – Like glass, the surface of enamelled steel is very hard, which means that it is extremely resistant to scratching, abrasion, impact and wear. Hardness of the enamelled steels is between 5 and 7 on the Mohs scale. One of the advantages of the surface hardness of enamelled steel is that it is extremely resistant to abrasion. Abrasion resistance is determined by means of a friction test.
High and low temperature stability – Because of the vitreous nature, enamelled surfaces have outstanding temperature stability. Some applications of enamelled steels require operating temperatures of around 450 deg C to 500 deg C. Enamelled steel can also be subjected to temperatures of minus 60 deg C without any adverse effects on the enamel.
Thermal shock resistance – Enamel coatings can withstand wide temperature differences in excess of 100 deg C without suffering any damage. Hence, it can be used in applications where there are such variations.
Fire resistance – A flame or any other heat source does not cause any damage to an enamelled surface. Further, enamel coatings do not produce any toxic fumes in the event of prolonged exposure to heat. The fire resistance of enamelled panels is classified as A1.
Hygiene and ease of cleaning – The smooth, hard vitreous surface of enamelled steel has no pores or cracks. This prevents the growth of bacteria and the accumulation of dust. Hence, enamelled steel can be used in sensitive areas. Further, enamelled steel is a food-grade material which does not give off odours. Also, enamelled surfaces are very easy to clean. The smooth, sealed surface of enamel and its exclusively mineral composition mean that commercially available solvents can be used for cleaning purposes, making it much easier and less expensive to clean.
Colour stability – Enamelled steel comes in an almost infinite range of colours, patterns and textures with a gloss, semi-matt or matt finish. In addition, it is possible to reproduce any image with extreme accuracy by screen printing, e.g. signs, posters, works of art or photographs. If the process is performed at a high temperature, these images last as long as the rest of the enamelled panel. Since the colours are created using mineral pigments, they display considerable stability over time. One particular feature is that they are not sensitive to UV radiations.
Uses of enamelled steel
Enamelled steels can be used both for indoor and outdoor applications. Domestic uses of enamelled steel include (i) sanitary wares, (ii) electric water heaters, and (iii) domestic appliances and cookware.
Enamelled steel is, in fact, the only material capable of withstanding the many stresses to which domestic appliances are subjected, particularly in cooking applications. Hence, it has become indispensable for specific applications where it is unrivalled because of its many useful properties. Some of these useful properties are (i) better resistance to scratching and abrasion, (ii) enamelled steel neither retains nor absorbs odours, hence it cannot impart them, (iii) resistant to products normally used in the kitchen, whether they be acidic or alkaline, (iv) excellent corrosion resistance, (v) flame-resistant and can withstand a high temperature, (vi) safe for contact with food and prevents the growth of bacteria, (vii) very resistant to steam, hence can be readily used for this cooking method, and (viii) outstanding aesthetic qualities.
Enamelled steel has many applications in construction. It can be used as a cladding for buildings or tunnels and in the interiors of public places, such as train and metro stations, airports and other buildings, as a wall-covering and for false ceilings, partitions and lifts. Enamelled steel is also an outstanding material for fitting out clean rooms.
Enamelled steel is the ideal solution for outdoor applications since it is weather and UV radiation resistant, with virtually unlimited scope for decoration. Enamelled panels are particularly suitable for separation walls or for cladding more traditional brickwork buildings. They are prefabricated to match the exact dimensions of the building. The panels can be installed in any weather.
The use of enamelled steel for lining tunnels is recommended, as it makes them easier and cheaper to clean, ensures better illumination and improves fire resistance.
Enamelled steel is a very popular choice for fitting out public places. Flame resistant, vandal-proof, easy to maintain and offering virtually unlimited scope for decoration, it is ideal as a wall-covering and for ceilings, partitions, lift cars etc. Since it is free of bacteria and is not affected by moisture, it is also the perfect solution in hospitals, clean rooms and sanitary systems.
Enamelled steel also has important applications in industry, even in the most corrosive atmospheres, e.g. in the chemical and agro-food industries. Its resistance to chemicals and to fermentation makes it an excellent lining for silos, chemical reactors, dryers, closed tanks and other water-treatment plant storage systems. Furthermore, because of its resistance to high temperatures and heat reflection properties, it can be used in columns and heat exchangers. Its resistance to corrosion caused by combustion gases also makes it an excellent material for the manufacture of flue linings and exhaust manifolds.
Enamelled steel is an ideal solution for indoor and outdoor sign and communication panels. The surface does not get damaged by urban pollution, weather, or UV radiations. It is fire resistant and offers a host of decorative possibilities, making it the best possible material for the most sophisticated graphic creations.
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