Anodische Korrosion von Blei in schwefelsaurer Lösung unter dem Einfluß von Perchlorsäure

Anodic corrosion of lead in a sulphuric acid solution under the influence of perchloric acid Research has been carried out into the influence of temperature and concentration on the corrosion of pure lead in sulphuric acid/perchloric acid solutions. With sulphuric acid concentrations below 2.16 Mol pr. litre, specimens with pre-treated surface show, in the presence of perchloric acid, in the temperature range from approx. ? 8° C to approx. + 15° C a kind of pitting corrosion which has a marked maximum around ? 4° C. With lead not subjected to a pretreatment which has the effect of removing the oxide film, pit corrosion is also encountered at room temperature in 1.652 m-H₂SO₄ with HClO₄ admixture, but this corrosion, too, is not observed at even higher temperatures. An interpretation is suggested which postulates the selective adsorption of perchloric acid at centres already present on the electrode when the electrolyte is introduced, and the desactivation of the latter through reaction with a different electrolyte constituent.     

Die Korrosion von Messing in Ammoniumchloridlösungen

Corrosion of brass in ammonium chloride solutions The corrosion behaviour of brass (Cu77Zn21A/2, with and without addition of As) has been investigated in 0.01 to 1 M ammonium chloride solution of PH 4 and 2, and in solutions 1 M in chloride ion, with variable Na⁺ and NH⁺₄ concentrations (total concentration 1 M in each case), at pH 4 and 2. While arsenium-free brass was attacked by dezincification in most cases, intercrystalline corrosion was found on As-containing brass. The latter type of corrosion is found already at As contents as low as 0.01%. In a medium 1 M in chloride ion intercrystalline corrosion is found at PH 2, irrespective of the ammonium ion concentration, while at PH 4 intercrystalline corrosion is found only at higher ammonium ion concentrations (practically 1 M). The intercrystalline corrosion was found to be due to the segregation of Zn and As respectively at the grain boundaries and to the influence of As on the stability of the CuCl^?₂ formed by the anodic reaction. The influence of the solution composition on the progress in time of the intercrystalline corrosion is explained.     

Korrosion von unlegiertem Stahl in sauerstoffreier Kohlensäurelösung

Corrosion of unalloyed steel in oxygen-free carbonic acid solution The investigation into the influence of dissolved carbon dioxide on the anodic partial reaction of iron dissolution in 0.5 M sodium sulfate solution has revealed, in agreement with data published in literature,

• 1 ) that the transfer of acid molecules to the cathode is the step controlling the rate of cathodic hydrogen evolution in oxygen-free solutions of little dissociated weak acids. The cathodic limiting current density is, consequently, a direct function of acid concentration and not of the pH value as in the case of strong acids.
• 2 ) that the anodic dissolution of iron in sulfate solution is catalyzed by carbonic acid which gives rise to a change of the dissolution mechanism, although this change is not reflected by a change of the cathodic protection potential (?0.85 V_CU/_CuSo4) which, consequently aplies to waters rich in carbonic acid, too.

     

Untersuchungen über den Einfluß von Schwefelwasserstoff und Schwefeldioxyd auf die Korrosion chemisch beständiger Chrom-Nickel-Stähle in schwefelsaurer Natriumsulfatlösung

Investigations into the influence of hydrogen sulphur dioxide on the corrosion of chemically resistant chrome nickel steels in a sulphuric acid chrome nickel steels in a sulphuric acid solution of sodium sulphate By measuring the current/potential curves and determining the weight losses, the influence of H₂S and SO₂ on the corrosion of an 18/8 Cr? NI steel and of an 18/8 Cr? Ni stell with 2pCMo and 2.8pC Cu in sodium sulphate solution has been investigated. H₂S and SO₂ have the effect of shifting the rest potential towards the electronegative side, compared with a solution flushed with nitrogen, enlarging the potential range of active dissolution, and greatly increasing the dissolution, and greatly increasing the corrosion rate in the active zone. From the results of the measurements, it may be concluded that the metal dissolution is catalyzed by hydrogen sulphide ions and probably by reduction products of the sulphur dioxide. This catalyzing effect is not confined to the zone of activation overpotential but also occurs in the zone of the active plateau, the shape of which is largely determined by the migration phenomena. SO₂also has the effect of increasing the dissolution rate in the passive condition.     

Die instationäre Korrosion des passiven Eisens in saurer Lösung

The instationary corrosion of passive iron in acid solution The corrosion rate of the passive iron, i.e. the dissolution rate of the passivating layer, in 1 n H₂SO₄ is investigated, with the layer thickness being increased and decreased, respectively (non-stationary conditions). The corrosion rate is determined chemical-analytically as well as coulometrically on the basis of galvanostatic measurements. Both methods yield the same result. The corrosion current density, i_K, increases as the formation current density, i_S, of the layer increases, and decreases the more below the stationary corrosion value, i_K, the higher the layer reduction current density (i_S <0) is. i_K and i_S, are independent of the electrode Potential of the passive iron electrode, similar to the behaviour of the stationary value, i_K,0. This independence is explained by the semiconductor properties of the passivating oxide (γ-Fe₂O₃). There is an unique correlation between i_K and i_Ss which can be explained also theoretically by a variable overvoltage at the phase boundary oxidelectrolyte. The kinetics of the corrosion and of the layer formation is evaluated from the values of the apparent charge transfer coefficients by referring to the p_H dependence of the stationary corrosion.     

Einfluß der Zusammensetzung von Messinglegierungen auf die Entzinkung und interkristalline Korrosion in Ammoniumchloridlösung

Influence of the composition of brass on dezincification and intercrystalline corrosion in ammonium chloride solutions The intercrystalline corrosion of As-containing α-brass is not restricted to a defined brass composition but is found also with brass types containing, in addition, AL and Sn. In the case of two-phase alloys of this type the β-phase is preferentially corroded, in Particular when the brass contains As and the β-phase forms a coherent network. On the basis of the results obtained a hypothesis has been derived as to mechanism the partial processes; this hypothesis is confirmed by result obtained with macroelements of As-containing and As-free brass. According to this hypothesis the intercrystalline corrosion of α- brass can be ascribed to the inhibition by As of the dezincification, so that no regions more anodic than grain boundaries are formed. This is why the boundaries are attacked preferentially.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

Die Kinetik der Auflösung von Aluminium in alkalischen Lösungen

The kinetics of the dissolution of aluminium in alcaline solutions The stationary dissolution rates of rotating aluminium discs were measured in alcaline sodium sulfate solutions as function of the Ph- value and of the rotational speed using the Förster eddy current test and electrochemical methods. The rates are controlled by diffusion or by heterogeneous dissolution of aluminate from the passive, oxide-covered aluminium. If the rate is diffusion-controlled, a secondary layer very probably consisting of hydrargillite is formed on the passive aluminium. The rate of the heterogeneous dissolution grows little with the Ph-value and depends strongly upon the electrode potential at unnoble potentials and weakly at noble potentials. Cathodic evolution of hydrogen increases the diffusion-limited as well as the reaction limited dissolution rates because of the excess of hydroxyl ions formed at the surface.     

Untersuchungen über die Kathodenwirkung von Eisensulfid und Mangansulfid auf die Korrosion von Eisen in dreiprozentiger Natriumchloridlösung

Studies on the cathodic effect of iron sulfide and manganese sulfide on the corrosion of iron in a three percent sodium chloride solution In the galvanic element iron/sodium chloride solution/iron sulfide and in the element iron/sodium chloride solution/manganese sulfide, flushed with nitrogen or air, the sulfides become the cathode and the iron is anode. Upon short circuiting the electrodes a strong cathodic polarisation of the sulfides occurs. However, only if the iron is in contact with iron sulfide in the air-flushed sodium chloride solution the corrosion of the iron is enhanced because of considerable anodic polarisation. The corrosion of the iron electrode is increased corresponding to the increase in cathodic area. According to these results inclusions of iron sulfide or manganese sulfide act as local cathodes but cannot cause pitting corrosion of unalloyed steels in a neutral chloride solution.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

Untersuchungen zum Einfluß von Sulfid- und Sulfationen auf die Korrosion des Eisens in alkalischer Lösung

Investigation into the influence of sulfide and sulfate ions on the corrosion of iron in alcaline solution The corrosion behaviour of Armco iron as well as that of a technical high-strength steel was investigated in sulphide - and sulphate-containing electrolytes of pH ～ 12.6. Current density - potential curves show that iron, after successful passivation, remains passive and protected against corrosion in a solution saturated with CaS, CaSO₄ and Ca(OH)₂ at potentials up to 800 mV(EH). Passivation experiments conducted by changing from the active region to various anodic potentials yielded a critical potential region around 300 mV(EH), above which corrosion appeared. Up to E_H = 200 mV complete passivation was observed. The same critical potential region was observed in experiments of repassivation after mechanical damaging of a protective passive layer. Additional tensile stresses of 85%· σ_0,2 caused no stress corrosion cracking at a potential of 500 mV(EH). After damaging the passive layer, through scratching of the stressed wires fracture occured at and above potentials of 300 mV(EH). At 200 mV(EH) repassivation was observed even under tensile load. In a sulphate-free Na₂S/Ca(OH)₂ solution of the same sulphide ion concentration no stress corrosion cracking of the samples occured at any potential up to 800 mV(EH) even after scratching the steel samples. The observed stress corrosion in sulphide - and sulphate-containing electrolytes is to be ascribed to the action of sulphate and not to sulphide ions.     

Zur Korrosion von Lötverbindungen

Corrosion of brazed joints A soldered electrical PtNi contact was attacked in air with formation of Pb (OH)₂ · (PbCO₃)₂. This corrosion product, because of its dielectric properties, deteriorated the contact performance of the Pt? Ni couple. It is therefore recommended not to use lead containing alloys for soldering electrical contacts.     

Korrosion von Chemieemail in alkalischen Lösungen

Corrosion of chemical service glass-enamel in alkaline solutions Chemical service glass-enamels are high resistant, multi component silicate glasses fused onto steel yielding a compound material of mechanical and chemical stability as well. The resistance of these glass-enamels against alkali attack is investigated in laboratory tests involving gravimetric evaluation of totally glass-lined steel samples after contact with various alkaline media. Thus the influence of the following technically relevant parameters on the glass corrosion rate becomes perceptible: agitation and renewal of the medium, volume-to-surface ratio, exposure time, temperature, alkali content, glass quality; special attention is paid to inhibition effects originating e. g. from calcium(II) ions. These results may help to conceive on a laboratory scale a corrosion test for glass-linings which simulates in good approximation service conditions. In consequence, the corrosion situation in glass-lined equipment becomes more evident and appropriate measures may be considered to improve its economy.     

Die Korrosion des Bleis in tetrafluoborsauren Elektrolytlösungen

The corrosion of lead in acid tetrafluoborate electrolytes The corrosion of 99,985% lead in acid tetrafluoborate electrolytes, predominantly 1 mole/litre HBF₄ and 2 mole/litre Pb (BF₄)₂, was investigated. The corrosion rate in air saturated solution at 20°C is 35 μA/cm², which corresponds to 1 mm/a. Oxygen corrosion predominates (corrosion is rather reduced under argon). At higher temperatures, corrosion rate increases by an order of magnitude, and acid corrosion has a greater share due to a decrease of hydrogen overvoltage on lead and solubility of oxygen. Cementable additives like Cu⁺⁺, Sb⁺⁺⁺ and Pd⁺⁺ are found to enhance the initial corrosion rate by up to two orders of magnitude. However, after 10–20 hours, the original value is reestablished. Bi⁺⁺⁺ is cemented as well; the resulting acceleration of corrosion is proportional to the additive concentration and decreases but slowly. Oxidants like Fe⁺⁺⁺ or quinone accelerate the dissolution of lead as well and are consumed stoichometrically. Reducing agents like Fe⁺⁺, V⁺⁺⁺ or hydroquinone are not able to act as a mediator for air corrosion. Nitrate ions are virtually inert in the given concentration range. Consequences in reference to the lead dissolution secondary battery are discussed. Self discharge in the absence of O₂ is very low. On the other hand, it is possible by oxygen corrosion to redissolve lead, which has accumulated on the electrode. In this way, the battery can be brought back to its original state.     

Inhibierung der Korrosion von Kupfer in Natriumhydroxidlösungen

Inhibition of copper corrosion in sodium hydroxide solutions The authors have investigated the effect of 30 organic substances on the corrosion of copper in aqueous NaOH solutions (0.2, 0.5 and 1.0 n). The following substances were found to be effective inhibitors: resorcin, phloroglucin, pyragallol, tanni, β-naphthom, m and p-aminophenol, glucose, furfural, cyclohexanon, 8-oxychinolin, hydrazine sulphate, chinalizarine, hydrochinon, sodium diethyldithiocarbamate, sodium rhodizonate, gallocyanine and acriflavine. In most cases, the inhibition effect is due to a covering layer of reaction products of metal, inhibitor and hydroxide. The decisive factor is the stability of the covering layer with the different lye concentrations.     

Korrosion von CuZn-Legierungen in Chloridlösungen bei potentiostatischer Polarisation

The corrosion of CuZn alloys in Chloride solutions with potentiostatic polarisation The intercrystalline corrosion susceptibility of brasses can be studied either by potentiostatic methods in NH₄Cl+HCl solutions with polarisation to +80 mV, or – without polarisation – in NH₄Cl+HCl+CuSO₄ solutions where the potential of +80 mV is attained without external polarisation. Tests carried out with macroelements formed either by brasses having different structures (α, β/α), or identical structure (α) but different zinc contents (21 and 32% respectively) have shown the preferential attack of the β phase at potentials lower than those required for grain boundary dissolution, and the more anodic character of the alloys of higher zinc content.     

Inhibierung der Korrosion von Kupfer in salzsauren Lösungen

Inhibition of copper corrosion in hydrochloric solutions Furfural is one of the best corrosion inhibitors of copper in hydrochloric acid. The inhibition effect ranges from 90 to 100 per cent. With increasing acid concentration, the quantity of furfural required for effective corrosion inhibition becomes smaller. By means of polarisation measurements, it can be found that, in 0.2 n and 1.0 n HCl, the inhibitor has an essential anodic effect and the cathode becomes depolarised whilst, in 2.0 n and 3.0 n HCl, the anodic zone is virtually unchanged and there is a considerable polarisation in the cathodic zone. The protective effect of furfural is probably due to its resin forming capacity.     

Die Abhängigkeit der Korrosion von Kohlenstoffstahl von der Zusammensetzung des Kühlwassers

Relation between the corrosion of carbon steel and the composition of cooling water The rate of uniform corrosion of carbon steel depends on the calcium, magnesium and bicarbonate ion content, the pH and the temperature of the cooling water. This dependence has been expressed as a mathematical relation which yields an accuracy of ±30% for longterm measurements. On principal it appears feasible to formulate the relation between corrosion and pH_s and I_s but the error would be significantly increased. So far it has not been possible to find a relation between the cooling water composition on the one hand and the formation of shallow grooves and corrosion pits on the other.     

Zur Korrosion von Reinaluminium in strömender Salpetersäure

Corrosion of pure aluminium in flowing nitric acid After 10 to 15 years in use under operating conditions, piping systems made of Al-99.5 in which highly concentrated nitric acid was flowing at a flow rate of 1 m/s exhibited flow-induced corrosion patterns. In corrosion tests involving single phase flow, no relationship could be found between flow and corrosion of Al-99.5 in nitric acid of various concentrations. The following testing methods were used: rotating disks of various configurations, rotating cylinders, and flow inside a tube. The influence of two-phase flow generated by a special mixing turbine resulted in weight losses of Al-99.5 with respect to nitric acids having concentrations of 90 to approximately 100%. In the Arrhenius diagram, the temperature dependence of the data with practically represent anhydrous nitric acid is shown as a scatter band. Lower nitric acid concentrations are shifting the scatter band in the direction of higher corrosion rates.     

Korrosion und kathodischer Korrosionsschutz von unlegiertem Stahl in Sandböden

Corrosion and cathodic protection of unalloyed steel in sand soil Cathodic protection takes place if the potential criterion is satisfied. That means that the pipe-to-soil potential is less than the protection potential. This is to be controlled in the case of pipelines which are required to be inspected regularly by law. Protection potentials are described in technical standards (e.g. DIN 30676), and must be determined experimentally in doubtful cases. In general the protection potential is referred to as U = ?0.85 V for soils. In practical applications of this protection potential difficulties arise in the case of sand soils with a minute amount of salt and a high resistivity resulting from a very low humidity. These difficulties are related to a high protection current demand due to good aeration and a poor current distribution due to the high resistivity. On the other hand, sand soils are known to be less corrosive. Thus, one can think of using special protection potentials for sand soils which are more positive than ?0.85 V. The results of laboratory and field tests show that protection potentials in the range of ?0.75 V to ?0.65 V can be applied depending on soil resistivity. In these cases even micro cell action can be prevented.