Accumulated statistics show that worldwide annually around 30% of all metal constructions become useless as a result of damages caused by corrosion. Corroded metal equipment is taken out of operation and subjected to processing in corresponding metallurgy plants. A certain part of metal equipment is subjected to electro chemical corrosion in electrolytic (so-called corrosive) medium. Included here are tanks designed for storage of acids, bases, salts, etc., which are subjected to electrolytic corrosion as they are surrounded by an electrolytic medium. Nowadays, there are multiple possibilities to fight corrosion effectively. There are
design phase of the metal equipment by selecting the suitable material. Depending on the conditions and properties of the medium that the metal equipment is going to be subjected to various brands of steel and alloys having the appropriate corrosion resistance should be used to make it reliable and inexpensive. This task is covered by the present assignment of selecting materials for storing diluted sulphuric acid and hydrochloric acid (10 % w/w).
5.5.2 Anodic passivation
It was M.V.Lomonosov (1738) and later Bertselius and M.Faraday that described the paradoxical fact that iron easily dissolves in diluted nitric acid solutions but this process continues only until acid concentration is increased. Faraday explained this by the formation of a protective oxide layer on the iron surface, which prevents the metal from dissolving further. The process of bringing metals to a condition of increased resistance when subjected to various oxidising agents is called passivation. With all the practical application possibilities that this phenomenon offers, namely for fighting metal corrosion, it has been the target of multiple and comprehensive studies ever since it was initially discovered.
In addition to being subjected to oxidising agents metals could also be passivated by means of anodic polarization, i.e. by using electric current to divert the equilibrium potential of the electrode in the positive direction. In this case we talk about anodic passivation.
Modern studies on anodic passivation of metals run basically in two main directions. On one side, the mechanics of the processes leading to changes in the condition of the metal surface and bringing the metal into stable (passive) state are investigated and the nature and morphology of oxidised layers thus formed are studied, on the other.
5.5.3 Mechanics of anodic passivation
Dependent on the nature of the solution and the value of diversification of electrode potential in the positive direction metal passivation could take place in different ways. In active anodic dissolving of metals there may be a point of saturation of the electrolyte with respect to certain metal salts, thus forming a solid product that can be deposited on the surface of the metal. If the apparent current density is maintained constant then the actual current density in the areas accessible for the electrolyte will increase as a result of the shield formed on the electrode surface by the deposited non-conductive solid product. The potential in these areas strongly diverts into the positive direction, which makes it possible for a thermodynamic reaction to take place and formation of a hard metal oxide or hydroxide. The oxide layer thus formed on separate areas eventually spreads on the entire metal surface preventing it from dissolving further.
Also outstanding are V.Muller’s (1933) trials that subjected iron, zinc, copper, etc. metal electrodes horizontally positioned in the electrolyte to anodic polarization in diluted sulphuric acid solutions. The polarized light studies have indicated that when there is no motion in the electrolyte hydrated sulphates of corresponding metals deposit on the surface of the electrodes. Once these salts are formed metals transform into their passive state.
Anodic passivation of metals could also be carried out without the formation of the shielding solid products. If no anions are present in the electrolyte to precipitate the metal cations that have entered the solution and if the potential of the electrode has been sufficiently diverted
the initial metal surface and the nature of the anodic reaction. Under such conditions the metal stops to dissolve measurably and transfers into passive state.
Theories of passivity of metals. Wide practical applications uncovered by metal passivation have enhanced not only applications studies in this area but also investigations involving the nature of the processes taking place in this phenomenon. A large number of hypothesis were made with regards to the mechanism of passivation and the nature of the passive state of metals. Various concepts of above issues could be classified into two basic groups. One of the groups, the smaller one, comprises the concepts according to which the passivation phenomenon is the result of a number of changes taking place in the physical properties of the metal itself (for example, the change in the electron state of the metal and its transformation into a specific, chemically inactive allotropic state). Another group of theoretical concepts links passivation to the formation of protective oxide layers on the metal surface. The metal does not become thermodynamically nobler during passivation but transfers into a stable state thanks to the protective layers formed on its surface, which significantly change the electrochemical properties of the metal-medium interface. And if the discussions on the issue of the passive state of metals still continue to be fierce they are mainly concentrated on the question: what is the nature of these protective layers? Are these two-dimensional layers of adsorbed oxygen that block the metal surface making it inactive (the adsorption theory) or these are three-dimensional oxide layers, which under the form of a separate phase cover the metal surface and purely mechanically prevent it from the actions of the corrosive medium (the phase theory).
On the modern stage of development of our concepts of the mechanism of passivation and the nature of the passive state the widest popularity has the phase theory the basic ideas of which were stipulated by M.Faraday (1836). U.Evans gave direct experimental proof in support of these theoretical conceptions. He succeeded to separate from the surface of passivated iron some oxide layers and examined them directly under the microscope.
5.5.4 Experimental studies – anodic protection of mild steel in sulphuric acid solution
Experimental setup. The apparatus consists of a simple support for suspending two electrodes made of mild steel strip into a 600 ml beaker containing approx. 400 ml acid. The current supply is from a stabilized power supply source.
The current and voltage values could be measured using the power source devices and a multimeter. We increase the voltage in 0.1V intervals up to maximum 3.0V and we measure the current value for every voltage increase. Before we take down its value we wait for the current to stabilize. The current readings increase steadily up to the point where the passive section is reached when it suddenly drops. When 3.0V is reached we reduce the current through 0.1V intervals to see if there are some hysteresis effects. We repeat the experiment adding to the acid a small quantity of concentrated brine solution. We plot the voltage variation in relation to current density logarithm and we analyse the results.
Analysis of resulting diagrams: The diagram shown in Appendix 1indicates that the curve consists of several characteristic sections, each corresponding to a specific metal state. We can note that when we increase the voltage current density suddenly increases from 1.5 to 3.0 log CD. This section corresponds to the active anodic dissolving of iron in sulphuric acid. For voltage values between 0.5V and 1.3V there is a section that characterizes by the fact that the current is independent of the potential of the electrode. A shielding solid product from basic iron sulphate is formed in this section, which has porous structure and dissolves in the electrolyte at a certain speed. When the speeds of formation and dissolving become equal the thickness of the sediment becomes constant and independent of the electrode potential. In the 1.3V section anode current suddenly drops and the electrode goes into passive state. In the section between 1.3V and around 2V iron is passivated. We could say the current is independent of electrode potential. Despite of the fact that the metal is in its passive state some very low anode current still flows through the electrode, which is due to the dissolving of passivated iron. This current is one of the most characteristic features of metals in passive state. The potential corresponding to this current could be regarded as the thermodynamic limit beyond which the metal goes in a state of active dissolving and above which conditions are created for forming protection oxide layers. After a voltage of 1.75V anodic deposition of oxygen starts on the passivated electrode as a result of which the current increases with every increase of electrode potential.
The opposite processes take place when current density reduces. For voltage values of 1.75V electrode potential changes in the negative direction and anodic metal dissolving is resumed.
In this process resumption the diagram indicates a hysteresis, which could be explained by the fact that the passive layer thus formed stays stable even at lower current densities than those when the layer was formed. It takes some time to cover the entire electrode with the oxide layer and this time would be smaller for higher anode current densities.
When we repeat the experiment after adding a small quantity of concentrated brine solution to the acid the acid saturates with positive hydrated ions of hydrogen and negative ions of chlorine, which are directed to the electrodes and ensure increase and eventual flow of direct
Fig. 30 Experimental setup diagram
Conclusions and practical applications. The results show that this experiment could be used for providing anodic protection of metals against corrosion. An oxidised layer covers metals when immersed into suitable oxidising medium (oxidation) or when used for a certain time as anodes in an electrolytic bath where their surface is oxidised by the oxide deposited on the anode (anodising).
5.5.5 Selecting materials for the tank
Expensive special steels (£ 3 per kg). High-alloy steels are being produced for industrial, building and domestic applications, which are both corrosion resistant and fireproof.
Corrosion resistant steels are capable of resisting the destructive chemical and electrochemical action of external environment. Considering steel’s ability to resist a certain aggressive corrosive medium it is classified as stainless, acid resistant and scale resistant. Acid resistant steel exhibits high corrosion resistance against the action of various aggressive mediums.
Corrosion resistant steels are usually chrome- or chrome-nickel alloys containing above 12%
chrome. Depending on its chemical composition steels microstructure could be ferrite, semi-ferrite and allowing structural transformations, i.e. that can be subjected to improvements (above 15% carbon, 10% to 18% chrome) having austenitic structure.
High nickel and manganese steels feature extended areas of stable austenitic structure. When chrome-nickel steels are heated up to temperatures of 490 – 900 degrees or when being cooled down slowly in this interval chrome carbides are formed along the boundaries of the austenitic grains. This results in grains of rich in chrome and poor in carbon cores. As a result of this structural non-uniformity steel shows a tendency for inter-crystal corrosion. To avoid this drawback chrome-nickel steels have to be additionally alloyed using strong carbide-forming elements such as titanium and niobium.
The austenite structure of chrome-nickel and chrome-nickel-manganese steels renders these materials some very essential properties, such as non-magnetic characteristics, improved strength under high temperatures and good weldability.
Chrome-nickel steels usually acquire satisfactory strength and good plastic characteristics after being hardened with austenite. Strength characteristics of such steels can be improved by cold-work hardening by means of cold rolling, cold drawing or stamping. Cold-work austenite steel maintains sufficient plastic properties. Semi-finished products made of such steel could be bended, shaped or even stamped.
Selecting a high-alloy steel brand. For our application I select X5CrNiMoCuNb18 18 steel, which exhibits highest sulphuric and other acids resistance and finds wide application in the chemical industry. I am going to give the following most essential mechanical characteristics for this steel: the strength σB, the yield strength σS, the percentage of specific elongation δ, the percentage reduction of area ψ and impact strength
a
K. I will also apply the chemical composition, approximate forging and temperature treatment temperatures. The data is presented in the following tables:Steel brand σS σВ δ ψ аK
ISO DIN МРа МРа % % KJ/m2
X5CrNiMoCuNb18 18 1.4505 230 750 40 35 650
Steel brand Content of elements in %
ISO DIN C Si Mn Cr Ni others
X5CrNiMoCuNb18 18 1.4505 <0,07 <1,0 <2,0 16,5-18,5
16,5-18,5
Mo=2,0-2,5 Cu=1,8-2,2 Nb>8x%C
Steel brand Forging Hardening
ISO DIN Temp. Coolant Annealing
negative
Temp. Coolant
Tempering
X5CrNiMoCuNb18 18 1.4505
1150-750 air -
1050-1100 water -
Low-cost steels (£ 1 per kg). Good quality carbon steel is intended for the needs of all machine-building industry areas. Parts made of this type of steel are usually subjected to temperature and thermo-chemical treatment. To meet variable and often stringent requirements in this industry these steels contain basic components that compared to regular carbon constructional steels have tighter limit deviations, smaller quantities of harmful impurities, more uniform structure and higher non-metallic inclusions purity.
The basic properties and main purpose of these steels are determined by the carbon content.
Low-carbon steels (C<0.25%) do not exhibit high strength but have better plastic and ductile characteristics. These are usually used for making parts involving bending, drawing, roughing, stamping and welding.
Medium-carbon steels containing above 0.25% to 0.60% carbon exhibit sufficient strength combined with good ductility. These are mainly used for making parts involving forging, hot stamping and cutting. Lower carbon content gives steel good weldability and higher carbon content provides medium to poor steel weldability.
High-carbon steel containing above 60% carbon exhibits high strength, hardness and satisfactory ductility characteristics. This is usually used for making springs and parts demanding high wear characteristics.
Compared to alloy steel carbon steel has the advantage of being the cheapest good-quality steel but it features the following disadvantages:
• it features shallow hardness penetration so it is only suitable for small-diameter parts or thin-wall components;
• exhibits lower yield strength, fatigue strength and impact plasticity and ductility at equal tensile strength;
• hardness and strength of hardened steel quickly drops with temperature.
For our application we could select good-quality carbon steel containing up to 0.3% carbon (Ck22 steel), which features not so high strength but high ductility and very good weldability.
I am going to give the most essential mechanical characteristics, technological properties and
Steel brand
σ
Sσ
Вδ ψ а
KISO DIN МРа МРа % % KJ/m2
Ck22 1.1151 245 412 25 55 -
Steel brand Technological properties
ISO DIN Processing
involving cutting Weldability Forging temperature interval deg C
Cold processing
ductility
Ck22 1.1151 satisfactory. Very good 800-1300 Very good
Steel brand Temperature treatment
Hardening conditions HRC hardness after relaxation
ISO DIN
Temp deg С Coolant
HRC hardness after hardening
200 deg С
Ck22 1.1151 900-920 water 34-40 32-36
Because the selected steel is not acid-resistant for the application that we are going to use it we shall need to provide some additional protection of the metal against corrosion in the aggressive medium. We could apply passivation of the metal surface for the purpose (cover the metal with thin oxidized layer) or use protection methods as described in detail under Para VII.
The materials I selected exhibit good plastic properties allowing the tank to be designed and made in the most suitable and optimum cylindrical shape. Both steel brands feature good weldability, which allows for the tank to be made as a welded construction guaranteeing its surface uniformity.
The tank made of the selected brand of alloy steel X5CrNiMoCuNb18 18 could be used for storage and transportation of 10% hydrochloric acid (HCl) as for this acid this type of steel has good corrosion resistance. The tank made of Ck22 steel would not allow for storage of hydrochloric acid without additional protection. In this case we could apply cathode protection (electrochemical protection) or inhibitor protection (adding admixtures to the hydrochloric acid to stop the corrosion process). The following Paragraph provides more detail about these techniques.
5.5.6 Factors influencing the efficiency of the tank and its usage When designing the tank we should give consideration to all factors that influence its efficiency and affect its usage. Consideration should also be given to the efficiency of decisions made, to the possible manufacturing and assembly technology, to the operation and service conditions, to the maintenance and service life, reliability, etc.
One of the most significant factors defining the functionality, manufacturing and handling of the tank is its shape. The most suitable and optimum shape in this case is the cylindrical.
Bending steel sheet or joining individual rings can achieve this. Welding is a suitable
technique to apply for joining metal parts. The materials I selected feature good ductile characteristics allowing us to manufacture such type of construction. Both steel types have good welding ability and the tank can be made as a welded construction to guarantee its surface uniformity.
High efficiency could be achieved if universal devices and elements are used. Selected as such could be suitable handles for the tank, transportation wheels, drain valves, covers, level meters, etc.
The construction of the tank should not have excessive reserves (for strength, etc.). It should comply with the anticipated time for its service life. Nowadays, service life times for such equipment have been greatly reduced and the requirements for capabilities of operating in higher capacities, reliability, efficiency, convenience of operation, ease of maintenance, etc.
have increased. All these affect the selection for the metal and tank wall thickness.
The tank should comply with the requirements for aesthetic industrial and ergonomic design.
A study of available constructions should be made to help making the best decisions and introduce new solutions.
The design of the tank should comply with the requirements for transportation and handling providing clamping locations and ensuring means for moving the tank and draining and filling-in acid.
The tank should meet the reliability and handling safety requirements. A warning should be provided on the outside to indicate its contents and handling safety instructions should also be indicated.
5.5.7 Using the tank for storage of 10% hydrochloric acid.
The tank made of the selected brand of alloy steel X5CrNiMoCuNb18 18 can be used for storage and transportation of 10% hydrochloric acid (HCl) as this type of steel provides good corrosion resistance for this type of acid.
The tank made of 20 steel would not allow for storing hydrochloric acid unless some additional protection is provided. In our particular case cathode protection (electrochemical protection) or inhibitor protection (making additions to the hydrochloric acid to stop the corrosion process). Details on these methods were presented under Para V.