Sulfuric aqueous corrosion

Aqueous corrosion can occur even when the metallic object to be protected is ostensibly not immersed in water, if the relative humidity of the atmosphere exceeds 60%. In that case, a film of water will in fact be present on the metal surface. Further, if sulfur dioxide is present in the air, corrosion in the thin film of water will be greatly accelerated, partly because the acidity of the dissolved SO2 facilitates the oxygen absorption reaction... 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

Phase stability diagrams present, in graphical form, the thermodynamic data for the corrosion of metals in presence of sulfur and oxygen at a given temperature. In this sense, they are comparable to the -pH diagrams that present the thermodynamic data for aqueous corrosion. 

I. Epelboin, R Morel, and H. Takenouti, Corrosion inhibition and hydrogen adsorption in the case of iron in a sulfuric aqueous medium, J. Electrochem. Soc. 778 1283 (1971). 

STM has been used to study adsorption on surfaces as it relates to corrosion phenomena [6, 7]. Sulfiir is a well known corrosion agent and is often found in air (SO2, FI2S) and in aqueous solution as dissolved anions ( HSO7) or dissolved gas (FI2S). By studying the interaction of sulfur with surfaces, insights can be gained into... 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

The deleterious effect of sulfur dioxide and sulfites in domestic water is increased corrosivity owing to the lowered pH. However, oxidation of sulfite to sulfate in aqueous solutions uses dissolved oxygen, and lliis may retard corrosion. While llte oxichition of sulfite and sulfiirous acid to sulfate and sulfuric acid in the atmosphere is an environmental concern, this reaction is too... 

Corrosive wastes are acidic or alkaline (basic) wastes which can readily corrode or dissolve flesh, metal, or other materials. They are also among the most common hazardous wastestreams. Waste sulfuric acid from automotive batteries is an example of a corrosive waste. U.S. EPA uses two criteria to identify corrosive hazardous wastes. The first is a pH test. Aqueous wastes with a pH greater than or equal to 12.5, or less than or equal to 2 are corrosive under U.S. EPA s rules. A waste may also be corrosive if it has the ability to corrode steel in a specific U.S. EPA-approved test protocol. 

Traditionally, anodic oxygen evolution from acid solution—particularly from aqueous electrolytes containing sulfuric acid—has been performed at lead anodes that are passivated and stabilized against corrosion by a selfforming coating of Pb02. 

When sulfuric acid, H2S04, is added to water, what makes the resulting aqueous solution corrosive ... 

Despite the high reactivity and the low cost of the reagents, the use of sulfuric acid is however connected with some disadvantages. First of all, because it captures all the water liberated, it cannot be recycled directly without a high temperature dehydrating step (sulfuric concentration SC). If no SC is operated, the outcoming aqueous sulfuric acid has to be neutralized and produces highly salty effluents, non desirable for the environment. Furthermore, dilute sulfuric acid can produce severe corrosion problems of the reaction vessels. 

Although a patent was issued as early as 1936 covering aqueous phosphoric acid- - the sulfuric acid ester process was unchallenged for nearly 20 years in spite of corrosion and pollution problems, but is no longer in much use. 

The relation between the corrosion rate and pH in various aqueous media is depicted in Figure 4.5 and the corrosion rate is minimal in the pH range 4—8.5. Most of the alloys are inert to concentrated nitric and acetic acids, but are attacked by dilute nitric, sulfuric and hydrochloric acids. The sodium silicate acts as an inhibitor. The sensitivity to acid attack also depends upon the alloy. 

The wrought version of Haynes 6B alloy has higher corrosion resistance in aqueous media than the cast and weld overlay version (Stellite 6) because of hot forging and rolling breakdown of the interconnected carbides into discrete particles. Haynes 6B can be used in sulfuric acid in the concentration range 2-77 wt % at 24°C and in dilute solutions at higher temperatures (see Table 4.50). 

---