Black rust

There is an alternative to the formation of normal red rust described in reactions (2.3) to (2.5) earlier. If the anode and cathode are well separated (by several hundred millimetres) and the anode is starved of oxygen (say by being underwater) the iron as Fe will stay in solution. This means that there will be no expansive forces as described earlier to crack the concrete so corrosion may not be detected. 

This type of corrosion (known as black or green rust due to the colour of the liquid when first exposed to air after breakout) is found under damaged waterproof membranes and in some underwater or other water saturated conditions. It is potentially dangerous as there is no indication of corrosion by cracking and spalling of the concrete and the reinforcing steel may be severely weakened before corrosion is detected. Rebars may be hollowed out in such deoxygenated conditions particularly under membranes or when water is permanently ponded on the surface. 

Rust Staining on the concrete surface may be indicative of this type of attack, but obviously if water is getting under a membrane and excluding oxygen it is unlikely that the iron in solution will get to the concrete surface where it will then precipitate out to form rust stains. 

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Simultaneously, the temperature inside a polymerization reactor can rise to 1400 - 1500°C for some seconds (Fig. 7.2-7, bottom). The increase of the temperature is higher when the initial pressure and temperature are higher. The time during which the high temperature is maintained is only short. Black rust is not formed and the strength of the construction materials is not affected. 

Below the reddish layer, an electronically conductive layer of black rust, pictured as an arrangement of macropores, covers the metal except at the end of the pores. The flattened aspect erf the diagrams r cts the presence of this macroporous layer. 

The black rust is covered by a very compact microporous layer, made up of green rust and calcium carbonates. This film influences the high-frequency loop of the impedance diagrams. 

We will return to the subject of pitting corrosion later. It is related to the problems of coated reinforcement and to the black rust phenomenon discussed earlier. 

Particular problems arise when the corrosion problem is the black rust described in Chapter 2 and prestressed, post-tensioned structures where corrosion is difficult to detect as the tendons are enclosed in ducts. Tendon failure can be catastrophic as they are loaded to 50% or more of their ultimate tensile strength and modest section loss leads to failure under load. The particular problems of assessing prestressed concrete structures are addressed in Chapter 4. 

One problem with waterproofing membranes is that they have a 10 to 15-year life span. This means they must be replaced and any areas of concrete damage repaired. There is also a problem with severe pitting and black rust on the reinforcing steel as discussed in Chapter 2 and as shown in Figure 6.9. 

A different situation exists when high levels of water saturation and low levels of oxygen lead to the black rust that is deposited in the concrete without exerting stresses (see Section 2.2). In this case the corrosion rate measurement can be taken as showing a section loss that will eventually lead to failure. However, it is very difficult to extrapolate an instantaneous measurement of corrosion rate to a total section loss measurement. If we can make a series of corrosion rate measurements at different locations and we can come up with a compensation for variations in relative humidity and temperature we can estimate the average corrosion rate. We also have to estimate the original time to corrosion, and assume that the corrosion rate has either been constant or increased in some sensible manner (say linear or logarithmic) to the present condition. 

In oxygen-free hot water, steel is protected by formation of a natural coating of magnetite (Pe304 or black rust) formed by the reaction ... 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

This is a process to remove heavy black scale and rust from the surface. Hot-rolled sheets that may have such scale formation need only be acid pickled. Cold-rolled sheets, which may carry no such scales, need not be acid pickled. Depending upon the type of surface, one of the following methods may be adopted. 

Russ, m. soot lampblack, carbon black (Agric.) rust, -abscheidung,/. deposition of soot, -ansatz, m. deposit of soot, russartig, a. soot-like, fuli nous. 

Some pigments promote corrosion owing to their content of soluble salts, their reactivity, or their electrochemical action, and thus should be avoided. Rust of the spotted type can be the consequence of their presence in a paint, especially the hrst coat, e.g. of graphite (noble to steel), some red oxides of iron, gypsum, ochre or lamp black. 

The American Aeronautical Material Specification AMS 2480 A calls for 150h salt-spray test without rusting extending more than 0-125 in (3-175 mm) on either side of scratch marks, using a black enamel finish for the phosphate coating. 

The commonest staining trouble is iron stain —the blue-black stain caused by the interaction of soluble iron corrosion products and the natural tannins in wood. Hardwoods are generally more susceptible than softwoods. Steel wool should not be used for smoothing wood surfaces. Iron stains, if not too severe, can be removed with oxalic acid. Heavy contamination with soluble iron corrosion products usually results in migration and conversion to rust deposits in the wood. 

A form of rust. Magnetic, dark-gray-to-black form of iron oxide (Fe304) that forms a protective film on steel surfaces. 

All pipes and fittings are transported to the pickling process in which an overhead crane is used to lower them into an acidic pickle liquor solution that chemically cleans and etches the black oxide surface layer resulting in a clean, rust-resistant pipe. 

The coffee trees in the world are subject to a large number of diseases (2-4, 8, 73, 92) lists range up to 60 and more, and yet the disease possibilities do not always disturb plantation owners. Occasionally, as where the leaf rust attacks, or where koleroga (black rot) is serious, growers are on the lookout for diseases and are anxious about their future. There are local places where diseases are apparently more benign than in others, and this tends to dull the worry about disease losses in a country. 

A little tolerance is also to be advised. Plants are part of the natural world, and are inevitably going to be less than perfect a leaf spot here or a nibbled leaf there is not going to be life-threatening. The A-Z of Plant Problems (seepp.320-342) contains more detailed advice on specific problems affecting woody plants the entries on aphids, black spot, powdery and downy mildew, and rose rust are particularly applicable to roses. 

FUngal diseases Black, sunken patches on leaves farleft) are caused by the fungal disease downy mildew. Plok off Infeoted leaves and dispose of them Immediately next year, be sure to rotate crops. Leek rust Heft) need not spell disaster for your orops, but In future years, look for leek oultivars that offer some resistance to this fungal Infeotlon. 

In areas where pine trees are raised commercially, there may be restrictions on buying and planting black currants due to the potential for spreading white pine blister rust, a fungal disease that is passed back and forth between pine trees and some species of currants and gooseberries. Check with your university extension service before buying plants. 

Fe(OH)2 does not exist as a mineral. The Fe is divalent and the structure which is based on a hep anion array, is similar to that of brucite. Pure Fe(OH)2 is white. It is, however, readily oxidized, upon which it develops into greenish-blue, so-called, green rust or, on further oxidation, into black magnetite. 

Rust formed by atmospheric corrosion is often voluminous (Fig. 18.4) and visually appears as loose orange-brown or black masses. This type of rust is always a mixture of phases and frequently consists of two layers - magnetite at the iron/rust interface (as a result of reduced oxygen supply) with an outer layer of loose lepidocrocite and/ or goethite. Hematite is formed during high temperature aqueous corrosion and is also found in the passive layer (which forms at room temperature). 

The mineral on which I made the following experiments is found m the syenite on the island of Lovo near Brevig, Norway. It was discovered by the pastor Esmarck, son of Jens Esmarck, famous professor at the University of Christiania. It is the latter who sent me a specimen, asking me to examine it, because, On account of its high specific gravity, he believed it to be the earth of tantalum. This mineral is black, with no indication of crystalline form or texture, and looks exactly like gadolinite from Ytterby, the exterior presents sometimes a thin rust-colored surface layer (12). 

In addition to the black tarry exudate, some samples of TNT and Amatol contained the red particles of an expl compound which contained iron. It was shown that this material was the result of reaction of TNT and alcohol in the presence of iron. The iron came from corrosion of the shell by the exu-date, in the case of TNT, or by moisture in the AN. Several methods were introduced to prevent rusting of the shell, one of them by coating the inside of the shell with acid-proof paint, another was by controlling the moisture in the AN and still another by pouring on top of the Amatol some TNT as a 1 boos ter-surround . The purpose of the latter was to seal the hygroscopic AN from moisture... 

Metals tarnish when their surface atoms react with gaseous substances in the air. Oxygen is a highly reactive element, as we saw in the previous chapter, and it combines with iron to form the ruddy oxide compound we recognize as rust. Copper reacts with oxygen and carbon dioxide to form a greenish patina of copper carbonate. Silver resists the advances of oxygen but will slowly combine with sulphur compounds in the air to form black silver sulphide. 

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