Protection can be achieved in four ways, with many coatings functioning in more than one way: (1 ) A barrier coating that prevents the corrosive environment from contacting the metal.
(2) A sacrificial coating that corrodes while giving cathodic protection to the underlying metal. (3) An inhibitor coating that slows electrode reactions.
(4) An electrically resistive coating that stifles electrochemical corrosion cells. Paints fall into this last category.
Corrosion Inhibitors
An inhibitor is a chemical added to the corrosive environment in small amounts to reduce the corrosion rate. Some inhibitors interfere with the anode reaction, some with the cathode reaction, and some with both. They are usually used to prevent general corrosion but most are not effective in preventing localized attack, such as crevice corrosion, pitting, or stress-corrosion cracking. Inhibitors have a critical concentration that must be reached or exceeded for them to be effective, and in some cases to prevent them from making corrosion worse. Cathodic and Anodic Protection
Cathodic protection converts all anodic area on a metal surface to cathodes so that corrosion ceases. The protected metal has positive current flowing onto it from the electrolyte everywhere on the surface so that no current flows off. This result can be achieved in two distinctly different ways.
(1) By connecting a sacrificial anode to the metal that is to be protected.
(2) By applying an electric current from a separate power source, a technique called impressed-current cathodic protection.
Anodic protection, on the contray, makes the entire metal surface anodic—so anodic that the metal completely passivates. Obviously, then, this technique is limited to metals that can form protective passive films. Since passivated metals still corrode at a low rate, anodic protection almost, but not completely, stops corrosion.
(Selected from: Van Nostrand Reinhold, Corrosion Control, Samuel A. Bradford,New York, 1992. )
材料10
防腐
腐蚀问题可以通过以下方法解决： （1）选择抗腐蚀环境的材料 （2）给金属上防护层
（3）改变服役条件，例如温度，压力和周转率
（4）改变环境的化学过程，例如PH值，浓度，通气和杂质 （5）增加防腐剂
（6）通过阴极或者阳极保护变化金属的电动势 （7）修改设备或者系统的设计
（8）替代腐蚀（可行的选择）
上面所列的方法是公认的处理腐蚀问题的方法，但不是所有的场合都足以应对。尤其，腐蚀工程师经常不能改变服役条件或者化学环境。这些像海洋一样不能改变或者近似不能改
变，工业过程，工业过程中流动的相当流畅，任何的改变将会引起生产者的狂热。 许多的腐蚀问题都是由不正确的设计或者不正确的材料设计。然而，材料的正确选择能够克服许多的环境条件甚至是一些设计的不合理。
一旦工程师已经确认没有灾难的危险，决定哪种方法克服腐蚀经常是考虑经济形势的。 材料的选择
不锈钢经常是面对未知腐蚀环境的首要选择，因为这种合金的抗氧化范围非常的广泛，但是它们不能够抵抗强大的减少溶液剂，例如盐酸。不锈钢也能被腐蚀尽管它们称作不锈钢。根据冶金结构一般不锈钢分为五种（马氏体，铁素体，奥氏体，双相和沉淀硬化不锈钢），材料的选择不仅要考虑抗腐蚀性还要考虑强度和经济性。
工业上的纯镍有很高的抗腐蚀性，尤其是碱盐，与一些相似机械性能的钢材合并。镍和镍合金在食品工业上应用非常的广泛，也被选择用于氯化氢和碳水化合物。它们对于高温和压力腐蚀有非常好的抗性。
在标准的发动机力系里铝是一种非常有电抗性的，在两层面板的组成下，它在空气中马上反应，内部紧密，无组织氧化物，外部厚，可渗透氢氧化物。铝能够与空气共处的，能够抵抗许多的PH在4至9溶剂。强酸和中强酸可以破坏铝的结构。氯化物离子是相当有破坏性的因为它们会攻击铝薄膜的薄弱点和凹陷。许多氯化有机溶解剂和醇类能够严重的破坏铝合金，有时甚至引起爆炸。 保护层
保护层是保护就于机械和物理性能而选择的金属免于环境的腐蚀。涂层合金有很好的机械性能（通常是钢）且就花费来说比起选择更具抗腐蚀却昂贵的材料具有很好的实用性。 保护能够通过四种方法完成，很多的涂层就是通过不只一种方法完成的： （1）一个阻止外界与金属接触的保护层 （2）牺牲阴极金属以达到保护 （3）抑制剂降低电极反应
（4）电阻抗层扼杀电气化学腐蚀单元格。划分为这样四种
腐蚀抑制剂
抑制剂是加入腐蚀环境以达到减低腐蚀速率的化学物质。一些抑制剂阻碍阳极的反应，一些阻碍阴极的反应，一些两者都阻碍。它们通常被用于阻止一般的腐蚀但是对于局部腐蚀例如裂缝腐蚀，凹陷或者是严重的应力腐蚀。抑制剂有一个关键的浓度，必须要达到或者超过这个浓度时才能起作用，在一些情况下可以阻止它们发生更严重的腐蚀。
阴极和阳极保护
阴极保护是变所有的阳极金属区域为阴极以便发生腐蚀。保护金属有阳极电流通过电解质流入所以电解质表面没有电流流过。这个结果可以通过两种完全不同的方法获得。 （1）通过牺牲阳极用以保护金属
（2）通过应用电流分离电势，一种技术称为传输电流阴极保护
阳极保护，相反的，使整个金属的表面阳极化，所以整个的阳极全部的钝化。明显的，这种方法对于能够产生钝化的金属还是有限制的。因为，钝化金属依然是在低速率腐蚀，几乎大部分的阳极保护，不包含全部，是停止腐蚀的。
（选自Van Nostrand Reinhold，腐蚀控制Samuel A. Bradford,New York, 1992）
Reading Material 11
Chemical Industry
1. Definition of the Chemical Industry
At the turn of the century there would have been little difficulty in defining what constituted the chemical industry since only a very limited range of products was manufactured and these were clearly chemicals, e. g. , alkali, sulfuric acid. At present, however, many thousands of chemicals are produced, from raw materials like crude oil through (in some cases) many intermediates to products which may be used directly as consumer goods, or readily converted into them. The difficulty comes in deciding at which point in this sequence the particular operation ceases to be part of the chemical industry's sphere of activities. To consider a specific example to illustrate this dilemma, emulsion paints may contain poly (vinyl chloride)/poly (vinyl acetate). Clearly, synthesis of vinyl chloride (or acetate) and its polymerization are chemical activities. However, if formulation and mixing of the paint, including the polymer, is carried out by a branch of the multinational chemical company which manufactured the ingredients, is this still part of the chemical industry or does it now belong in the decorating industry?
It is therefore apparent that, because office diversity of operations and close links in many areas with other industries, there is no simple definition of the chemical industry. Instead each official body which collects and publishes statistics on manufacturing industry will have its definition as to which operations are classified as \when comparing statistical information which is derived from several sources. 2. The Need for* Chemical Industry
The chemical industry is concerned with converting raw materials, such as crude oil, firstly into chemical intermediates, and then into a tremendous variety of other chemicals. These are then used to produce consumer products, which make our lives more comfortable or, in some cases such as pharmaceutical products, help to maintain our well-being or even life itself. At each stage of these operations value is added to the product and provided this added value exceeds the raw material plus processing costs then a profit will be made on the operation. It is the aim of chemical industry to achieve this.
It may seem strange in textbook like this one to pose the question \we need a chemical industry?\question will provide (i) an indication of the range of the chemical industry's activities, (ii) its influence on our lives in everyday terms, and (iii) how great is society's need for a chemical industry. Our approach in answering the question will be to consider the industry9 s contribution to meeting and satisfying our major needs. What are these? Clearly food (and drink) and health are paramount. Other which we shall consider in their turn are clothing and (briefly) shelter, leisure and transport.
(1) Food' The chemical industry makes a major contribution to food production in at least three ways. Firstly, by making available large quantities of artificial fertilizers which are used to replace the elements (mainly nitrogen, phosphorus and potassium) which are removed as nutrients by the growing crops during modern intensive farming. Secondly, by manufacturing crop protection chemicals, i. e. , pesticides, which markedly reduce the proportion of the crops consumed by pests. Thirdly, by producing veterinary products which protect livestock from disease or cure their infections.
(2) Health, We are all aware of the major contribution which the pharmaceutical sector of the industry has made to help keep us all healthy, e. g. by curing bacterial infections with antibiotics,
and even extending life itself, e. g. ^-blockers to lower blood pressure.
(3) Clothing* The improvement in properties of modern synthetic fibers over the traditional clothing materials (e. g. cotton and wool) has been quite remarkable. Thus shirts, dresses and suits made from polyesters like Terylene and polyamides like Nylon are crease-resistant, machine-washable, and drip-dry or non-iron. They are also cheaper than natural materials. Parallel developments in the discovery of modern synthetic dyes and the technology to \them to the fiber has resulted in a tremendous increase in the variety of colors available to the fashion designer. Indeed they now span almost every color and hue of the visible spectrum. Indeed if a suitable shade is not available, structural modification of an existing dye to achieve this can readily be carried out, provided there is a satisfactory market for the product.
Other major advances in this sphere have been in color-fastness, i.e. , resistance to the dye being washed out when the garment is cleaned.
(4) Shelter, leisure and transport. In terms of shelter the contribution of modern synthetic polymers has been substantial. Plastics are tending to replace traditional building materials like wood because they are lighter, maintenance-free (i. e. they are resistant to weathering and do not need painting). Other polymers, e. g. urea-formaldehyde and polyurethanes, are important insulating materials for reducing heat losses and hence reducing energy usage.
Plastics and polymers have made a considerable impact on leisure activities with applications ranging from all-weather artificial surfaces for athletic tracks, football pitches and tennis courts to nylon strings for racquets and items like golf balls and footballs made entirely from synthetic materials.
Likewise the chemical industry's contribution to transport over the years has led to major improvements. Thus development of improved additives like anti-oxidants and viscosity index improves for engine oil has enabled routine servicing intervals to increase from 300 to 6000 to 12000 miles. Research and development work has also resulted in improved lubricating oils and greases, and better brake fluids. Yet again the contribution of polymers and plastics has been very striking with the proportion of the total automobile derived from these materials dashboard, steering wheel, seat padding and covering etc.—now exceeding 40%.
So it is quite apparent even from a brief look at the chemical industry's contribution to meeting our major needs that life in the world would be very different without the products of the industry. Indeed the level of a country's development may be judged by the production level and sophistication of its chemical industry.
3. Research and Development (R &. D) in Chemical Industries
One of the main reasons for the rapid growth of the chemical industry in the developed world has been its great commitment to, and investment in research and development (R & D). A typical figure is 5% of sales income, with this figure being almost doubled for the most research intensive sector, pharmaceuticals. It is important to emphasize that we are quoting percentages here not of profits but of sales income, i. e. the total money received, which has to pay for raw materials, overheads, staff salaries, etc. , as well. In the past this tremendous investment has paid off well, leading to many useful and valuable products being introduced to the market. Examples include synthetic polymers like nylons and polyesters, and drugs and pesticides. Although the number of new products introduced to the market has declined significantly in recent years, and in times of recession the research department is usually one of the first to suffer cutbacks, the commitment to R &? D remains at a very high level.
The chemical industry is a very high technology industry which takes full advantage of the latest advances in electronics and engineering. Computers are very widely used for all sorts of applications, from automatic control of chemical plants, to molecular modeling of structures of new compounds, to the control of analytical instruments in the laboratory.
Individual manufacturing plants have capacities ranging from just a few tons per year in the fine chemicals area to the real giants in the fertilizer and petrochemical sectors which range up to 500000 tonnes. The latter requires enormous capital investment, since a single plant of this size can now cost $ 250 million! This, coupled with the widespread use of automatic control equipment, helps to explain why the chemical industry is capital-rather than labor-intensive. The major chemical companies are truly multinational and operate their sales and marketing activities in most of the countries of the world, and they also have manufacturing units in a number of countries. This international outlook for operations, or globalization, is a growing trend within the chemical industry, with companies expanding their activities either by erecting manufacturing units in other countries or by taking over companies which are already operating there.
(Selected from: Alan Heaton, The Chemical Industry, 2nd Edition, Blackie &. Son Ltd. , 1997.
阅读材料11
化学工业
1、化学工业的定义
在上世纪之初，给构成化学工业制品定义时很简单的，因为那时的化学产品时很有限的，例如，强碱、硫酸溶液。现在，千上万的化学品从天然材料中生产出来的，如原油被加工成很多中间产品，可以作为消费品，或做一些转变成消费品。定义化学工业的困难就在于确定操作程序中的那一点是化学工业的部分，举个例子来说明这种左右为难的情况，乳化油漆可以含有聚合物（聚乙烯树脂）/聚脂（乙烯基醋酸纤维）。很明显的，人造聚乙烯树脂（或醋酸纤维）和它们的聚合物都是化工产品。然而，如果油漆的合成和配制中含有聚脂，它是化工的副产品，那这种它是属于化学工业产品还是油漆工业产品呢？ 出现那样的情况，是由于操作环境的多样性和很多地方同其他工业的紧密联系，是由于没有给化学工业简单定义。相反，每一个收集，出版相关制作工业资料剂书籍的部门，将会给那些操作过程一个简单的定义。在比较那些不同来源的统计信息的时候，这是很重要的。 2、 化学工业的需要
化学工业跟很多原材料的转变密切有关。如原油，首先要变成化工中间产品，然后被加工成很多各种各样的其他化工产品。这些都经常被用来作为消费产品，而另一些产品则制成药用产品，用于保护我们的健康。每一个阶段的操作价值都被加入到产品中，而且它提供的这些附加价值远远超过了原材料和过程的成本，这样操作流程就产生了利润。这也是化学工业的目的所在。
本文提到的这个问题“我么需要化学工业吗？这应该是很奇怪的问题。如果我们试着去回答这个问题将会得出：（1）化学工业领域很广（2）化学工业对我们的影响很大（3）社会对化学工业的巨大需求。这些需要包括什么呢？重要的有新鲜的食物（和饮料）和健康的需要。其它的还有服饰，住房，娱乐及交通运输。
（1）、食物。化学工业对食物生产的最大贡献至少体现在三个方面。第一，生产更多的