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Pickling

Pickling is a treatment of metallic surfaces in order to remove impurities, stains, rust or scale with a solution called pickle liquor, containing strong mineral acids, before subsequent processing, such as extrusion, rolling, painting, galvanizing or plating with tin or chromium. The two acids commonly used are hydrochloric acid and sulfuric acid. Pickling liquor may be a combination of acids and may also contain nitric or hydrofluoric acids.

Carbon steel is pickled usually by either sulfuric acid or hydrochloric acid. At one time, sulfuric acid was the pickling agent of choice[citation needed] for picklers running integrated steel works. Hydrochloric acid is chosen in more modern lines when bright surfaces, low energy consumption, reduced overpickling and the total recovery of the pickling agent from the waste pickle liquor are desired.

Process
Laser cutting, welding, and hot working leave a discolored oxidized layer or scale on the surface of the worked steel. This must be removed in order to perform many of the surface finishing processes. Acid pickling is used to remove this scale. Acid cleaning is also used for removing inorganic contaminants not removable by other primary cleaning solutions. Acid cleaning has limitations in that it is difficult to handle because of its corrosiveness, and it is not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels. The hydrogen from the acid reacts with the surface and makes it brittle and causes cracks. Because of its high reactance to treatable steels, acid concentrations and solution temperatures must be kept under control to assure desired pickling rates.

Similarly, pickling is used in jewelry making, after fluxing or soldering, in order to remove any scale and contaminants from the jewelry metal, which is often sterling silver, copper, or gold.

Waste products
Waste waters from pickling include acidic rinse waters, metallic salts and waste acid. Spent pickle liquor is considered a hazardous waste by EPA. Some plants, particularly those using hydrochloric acid, operate acid recovery plants where the mineral acid is boiled away from the iron salts, but there still remains a large volume of highly acidic ferrous sulfate or ferrous chloride to be disposed of. Since the 1960s total hydrochloric acid regeneration processes have reached widespread acceptance.[citation needed] The by-product of nitric acid pickling is marketable to a couple of secondary industries including fertilizers.

Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralization. The lime neutralization process raises the pH of the spent acid and makes heavy metals in the sludge less likely to leach into the environment. Today however, the some of spent pickle liquor can be recycled or regenerated on-site by steel manufacturers.

Alternatives to acid pickling
Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity. For this reason, a thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air. This oil film must later be removed for many fabrication, plating or painting processes.

A process was developed in 2003 known as smooth clean surface (SCS),[1] representing an alternative to acid pickling. In the SCS process, surface oxidation is removed using an engineered abrasive and the process leaves the surface resistant to subsequent oxidation without the need for oil film or other protective coating.

Subsequent to commercialization of the SCS process, the Eco Pickled Surface strip steel processing technology (EPS) has been developed as a more direct replacement for acid pickling. Whereas acid pickling relies on chemical reactions to accomplish removal of rust and other surface impurities, EPS technology employs "mechanical pickling" to completely remove all surface oxides and contaminants while conditioning the surface to achieve a more uniform texture. The EPS process is considered "environmentally friendly" compared to acid pickling and it imparts to carbon steel a high degree of rust resistance, eliminating the need to apply the oil coating that serves as a barrier to oxidation for acid-pickled carbon steel.

Pickle liquor
Pickle liquor is an acid solution used to descale or clean steel in various steelmaking processes. Typically, the acids employed in the pickling of steel are hydrochloric, nitric, sulfuric and hydrofluoric acids or combinations thereof. When the solution becomes spent, pickle liquor is considered a hazardous waste by EPA.

Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralization. The lime neutralization process raised the pH of the spent acid and made heavy metals in the sludge less likely to leach into the environment. Today however, the majority of spent pickle liquor is recycled or regenerated on-site by steel manufacturers.

酸洗  

通常指清洁金属表面的一种方法。
　　一般将制件浸入硫酸等的水溶液，以除去金属表面的氧化物等薄膜。是电镀、搪瓷、轧制等工艺的前处理或中间处理
　　(一)利用酸溶液去除钢铁表面上的氧化皮和锈蚀物的方法称为酸洗。氧化皮、铁锈等铁的氧化物(Fe3O4，Fe2O3，FeO等)与酸溶液发生化学反应，形成盐类溶于酸溶液中而被除去。酸洗用酸有硫酸、盐酸、磷酸、硝酸、铬酸、氢氟酸和混合酸等。最常用的是硫酸和盐酸。酸洗工艺主要有浸渍酸洗法、喷射酸洗法和酸膏除锈法。一般多用浸渍酸洗法，大批量生产中可采用喷射法。钢铁零件一般在10％～20％(体积)硫酸溶液中酸洗，温度为40℃。当溶液中含铁量超过80g/L，硫酸亚铁超过215g/L时，应更换酸洗液。常温下，用20％～80％(体积)的盐酸溶液对钢铁进行酸洗，不易发生过腐蚀和氢脆现象。由于酸对金属的腐蚀作用很大，需要添加缓蚀剂。清洗后金属表面成银白色，同时钝化表面，提高不锈钢抗腐蚀能力。
　　(二)为了消除硅藻土载体表面吸附，减少色谱峰拖尾，载体在使用前需进行酸洗或碱洗处理。酸洗是把载体用6mol/L盐酸浸煮2h或浓盐酸加热浸煮30min，过滤，用水洗至中性，烘干。酸洗可除去表面上的铁、铝、钙、镁等杂质，但不能除去硅醇基。酸洗载体适宜于分析酸性样品。
　　（三）酸洗常用的酸为：盐酸、硫酸、硝酸、磷酸、氢氟酸。在酸洗时务必加入酸洗缓蚀剂，防止酸对金属的腐蚀。