STAC a division of GuiDon sa : +32 15 253810 ES- T : [email protected] C Slameuterstraat 1 b Industrial : stacoat.comCoatings B-2580 Putte STA C Belgium Industrial Coatings VAT : BE 0453.727.693 IBAN : BE91 1032 2397 3876 Let STA Ckle those corrosion problems! ref : Metals and alloys Subject : Metals and alloys The number of alloys in general and steel grades in particular is vast and difficult to summarize consistently. In appendix an attempt is made to review the most common types. The table below gives a summary. Ferrous alloys Carbon Other alloy elements All Manganese Silicon Chromium Nickel Molybdenum Vanadium Titanium Copper Phosphorus Wolfram Cobalt Steel 0,2 - 2,1 % Carbon steel Low alloy steel < 1,5 % ✓✓ ✓✓ Medium alloy steel 1,5 - 10 % ✓✓ ✓✓ ✓ ✓ ✓ ✓ High alloy steel > 10 % Stainless steel ✓✓ ✓✓ Weathering steel (Corten) ✓ ✓✓ ✓✓ ✓ ✓ Tool steel ✓✓ ✓ ✓ ✓✓ ✓✓ Dual-phase steel ✓✓ ✓✓ Maraging steel ✓✓ ✓✓ TRIP steel Eglin steel Hadfield steel ✓✓ etc… Cast iron 2,1 - 6 % 1 - 3 % ✓✓ ✓✓ Explanation: ✓✓ = major element ✓ = minor element = sporadic element Galvanised steel Hot-dip galvanisation Electrolytic galvanisation Metallisation Cold galvanisation Cold selective galvanisation Other metals and non-ferrous alloys Other metals: aluminium, copper, tin, etc… Non-ferrous alloys: bronze (copper and tin), brass (copper and zinc), etc… Carbon steel Carbon steel (also called “standard steel”, “low and medium alloy steel” or simply “steel”) contains mostly iron, with a carbon content between 0,2 and 2,1 %. It makes up the bulk of the heavy industrial steel market but is given to fast corrosion if not protected by a coating system. When it is uncoated exposed to air and moisture its surface will react with oxygen to iron oxide (rust). That iron oxide film is active and accelerated further corrosion (or rusting through or auto-accelerated oxidation to iron oxide). The term "carbon steel" may also be used in reference to steel which is not stainless; in this definition carbon steel may include the 2 following alloy types. Low alloy steel Low alloy steel contains < 1,5 % of alloy elements (excl. carbon). Alloy elements most commonly used are manganese (Mn) and silicon (Si). Just as carbon they’re used to increase strength and hardness. Silicon is also a by-product of the steel-making process, it is used to extract oxygen from the steel. Low alloy steel is the most commonly used steel in the world. This is because it is relatively cheap and very good machinable. Medium alloy steel Medium alloy steel contains 1,5 to 10 % alloy elements (excl. carbon). As with low alloy steel, manganese and silicon are the common elements (e.g. 1,6 % Mn and 0,7 % Si). Also chromium (Cr), nickel (Ni), molybdenum Let S TA Ckle those corrosion problems! Metals and alloys.docx Page 1 of 4 Edition: January 2020 (cancels and replaces the preceding ones) S T A C Industrial Coatings (Mo) and vanadium (V) are often used. The combined impact of these elements in one type of steel is not so easy to determine because they can temper or amplify each other. Chromium is often used to make steel oxidation and corrosion resistant. It increases the hardness and abrasion resistance. It is commonly used in combination with nickel or molybdenum. Chromium in conjunction with molybdenum (so-called chromonen steel) gives steel high temperature and mechanical resistance. Vanadium is also commonly used in conjunction with the chromium and molybdenum because of its similar properties. Nickel has positive effect on steel at very high and very low temperatures. It is also commonly used to compensate a number of unfavourable characteristics of chromium. Galvanized steel Galvanized steel is carbon steel that has been coated with a thin film of zinc creating a sacrificial barrier between the steel (iron) and the environment. Zinc oxidises faster and easier than iron, it “sacrifices” itself over a distance of a few mm (cathodic protection of steel, avoiding creation of rust (Fe2O3)), to form a passive layer of zinc oxides (white powder). This is a continuous slow process by which, over time, particularly when the zinc oxide layer is removed regularly (e.g. by friction), all zinc oxidizes and the effectiveness disappears. Selective cold galvanisation is an exception. Hot-dip galvanisation Hot-dip galvanisation is the warm application of a zinc layer on steel structures by dipping them into a bath of liquid zinc at ± 460°C. Metallisation Metallisation is the warm application of a zinc layer on steel structures by spraying them with molten (hot) zinc. Electrolytic galvanisation Electrolytic galvanisation (electroplating) is the warm application of a zinc layer on steel structures by deposi- tion via electrolysis. The layer is thinner and much stronger bonded than with hot-dip. Cold galvanisation Cold galvanisation is the cold (ambient temperature) application of a special coating (e.g. STACGALVA), with a very high zinc content (dry layer > 92 %w zinc), on steel structures, with brush, roller or pistol. Selective cold galvanisation Selective cold galvanisation is the cold (ambient temperature) application of a special coating (e.g. STACPRIMER- U1-ZIMIO), with a high zinc content (dry layer > 65 %w zinc), on steel structures, with brush, roller or pistol. The zinc particles are encapsulated in a resin matrix and are only “activated” when needed, namely at the spot of a scratch. This zinc layer is not subject to continuous oxidation of the contact surface with the air (what happens with the other galvanizing techniques). High alloy steel High alloy steel contains ≥ 10 % alloy elements (excl. carbon). Best known are weathering and stainless steel. Weathering steel Weathering steel (trademark Cor-Ten), is a group of steel alloys which were developed to prevent the need for coating. The alloy elements are chromium (Cr), nickel (Ni), silicon (Si), copper (Cu) and phosphorus (P). It forms a stable rust-like appearance if exposed to the weather. The rust-coloured and very dense metal oxide layer prevents oxygen to react with the underlying steel, which slows down the corrosion drastically (except for pits filled with water, were the oxidation continues). Due to this metal oxide layer it is not necessary to paint the material. It is best not to use it for thin steel plates because it stays sensitive to corrosion. The reason is that the metal oxide layer forms again after it is eroded (e.g. sand, dust, hail or even acid rain). If the layer forms once again, the plate thickness decreases. If that happens repeatedly, holes will develop in the thin steel plates. It is also not resistant against wet leaves because they slow down the wet-dry cycles, speeding up the corrosion process. Let S TA Ckle those corrosion problems! Metals and alloys.docx Page 2 of 4 Edition: January 2020 (cancels and replaces the preceding ones) S T A C Industrial Coatings Stainless steel Stainless steel (inox steel, inox, high grade steel, refined steel) its major alloy elements are chromium (Cr) and nickel (Ni). Chromium can be used alone to make steel stainless but in most cases it is combined with nickel (e.g. 18 % Cr and 8 % Ni). Nickel compensates a number of unwanted effects of chromium. Stainless steel is resistant against corrosion and oxidation. This property is due to the reaction of chrome with (atmospheric) oxygen. This creates a passive chromium oxide layer on the steel. This layer is very thin and transparent. It consists of a network of chromium (iii) oxide, that is conductive for electrons but not for ions. As a result, the steel is corrosion resistant as long as the layer is intact. That is unfortunately not the case in e.g. a chloride solution, such as sea-water or chlorinated swimming pool water. The chloride ions cause pitting corrosion (pitting), which is very difficult to stop because the chloride ions concentrate in the pits. Addition of molyb- denum can produce resistance against chlorine. In order to improve the properties, either the carbon content is lowered (but this reduces the machinability) or titanium is added (but this reduces the weldability). Other high alloy steel types Tool steel: alloyed with large amounts of wolfram (tungsten), cobalt and/or other elements (e.g. vanadium) to maximize solution hardening. This also allows the use of precipitation hardening and improves the temper- ature resistance. It is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge. Dual-phase steel: which is heat treated to contain both a ferritic and martensitic crystal structures for extra strength. Maraging steel: is alloyed with nickel and other elements, but unlike most steel contains almost no carbon at all. This creates a very strong but still malleable steel. TRansformation Induced Plasticity (TRIP) steel: has a triple phase crystal structure consisting of ferrite, bainite and retained austenite. During plastic deformation and straining, without addition of heat, the metastable austenite phase is transformed into martensite. This transformation allows for enhanced strength and ductil- ity. It is a high-strength steel typically used in the automotive industry. Eglin steel: uses over a dozen different elements in varying amounts to create relatively low-cost steel for use in bunker buster weapons. Hadfield steel (manganese steel or mangalloy): contains 12–14 % manganese, is very resistant to abrasion and achieves up to three times its surface hardness during impact, without any increase in brittleness which is usually associated with hardness. This allows mangalloy to retain its extreme anti-wear and unique non- magnetic properties. Examples include tank tracks, bulldozer blade edges and cutting blades on the hydraulic rescue tools. Other ferrous alloys Cast iron: is an iron alloy with 2,1–6 % carbon and 1–3 % silicon and/or manganese. It’s made by melting raw iron (pig iron, coming from hematite ore) together with cokes and chalk in a cupola, induction furnace or drum furnace.