What is Corrosion??? METALS WANT TO CORRODE – they want to exist as oxide compounds because oxides contain less energy and are more stable!! Electrochemical reaction involving an anode and a cathode. Deterioration of a material because of reaction with the environment.

Based on the appearance of the corroded metal, corrosion may be classified as: Uniform or General Galvanic or Two-metal Pitting Intergranular Crevice Fatigue Corrosion Dealloying Fretting Others

Galvanic Corrosion: Possibility when two dissimilar metals are electrically connected in an electrolyte* Results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion. Refer to Galvanic Series The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode). Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials. Big Cathode, Small Anode = Big Trouble

A metal having higher position can replace (reduce) other metals that have lower position in the series. Or in other words, zinc will corrode faster than copper

Design for Galvanic Corrosion Material Selection: Do not connect dissimilar metals! Or if you can’t avoid it: Try to electrically isolate one from the other (rubber gasket). Make the anode large and the cathode small Bad situation: Steel siding with aluminum fasteners Better: Aluminum siding with steel fasteners Eliminate electrolyte Coating of anodic protection

Pitting Pitting is a localized form of corrosive attack. Pitting corrosion is typified by the formation of holes or pits on the metal surface. Pitting can cause failure, yet the total corrosion, as measured by weight loss, may be minimal.

Crevice Corrosion Intensive localized corrosion within crevices & shielded areas on metal surfaces Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints

How to Control Corrosion There are five methods to control corrosion:  material selection  coatings  changing the environment  changing the potential  design

Zinc Flake Coating What do we mean by the term Zinc Flake Coating? Zinc flake coating systems were originally developed in the USA in the 1960’s, so they have been about for some time, development is ongoing with new materials coming to market on a regular basis. Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings usually consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix.

Zinc Flake Coating Zinc coatings are composed mainly of a mixture of zinc and aluminum (75:5), conductive binder and lubricant as required to adjust the coefficient of friction. Base coating - the main task is to protect the steel substrate from corrosion thanks to the active dissolution of zinc. Zinc corrodes like the first, for example on scratches. Zinc is located in the coating in flake form which allows for very thin coatings with a thickness cca.5um Protective coating - the main task is to control the mechanical properties such as friction, torque, resistance to some chemicals, abrasion resistance, as well as increases the corrosion resistance.

Zinc flake systems - advantages  Highest cathodic corrosion protection  Thin coating system, 5-25 µm  Environmentally friendly, suitable for applications in the field of toys and drinking water  Fullfills EU Directive 2000/53/EG and 2002/95/EG, Chromium (VI)-free Worldwide available  Versatile to use  Efficiency - Saves costs  No hydrogen embrittlement due to application process

Design Considerations. Type of corrosion protection required – galvanic or barrier. Cathodic protection is provided by sacrificial corrosion of the zinc flakes. Joint fastening requirements – Co-efficient of friction range required. Zinc flake coatings give controlled co-efficient of friction values.

Design Considerations. Heat treatment grade of fastener or part - associated risk of embrittlement. Zinc Flake application causes no hydrogen embrittlement. Electrical Conductivity – subsequent ecote or earthing needs. Zinc flake coatings are “usually” conductive.

Zinc Flake Process Steps

Zinc Flake Application Methods

Coating application. Anochrome Group Current Equipment in Lancut Planetary Dip Spin Lines. Barrel Dip Spin Line.

Planetary Dip Spin Coaters Resolves the original industry problem of blocked recess drives commonly termed as recess infill. Many new finishes tend to require a greater coating weight than previous coatings. Planetary motion plants are the recommended option of many OEM’s to reduce levels of recess infill (see WX Recess infill)

Barrel Dip Spin Line Even distribution of parts during spinning is critical for a recess infill free even coating. Small material surface area to stop material atmospherical changes is essential. Quick paint change over times. Reduced basket down times due to cleaning. Ability to process long parts up to 285mm. All the above points are a benefit over small basket planetary motion plants.

Dip Spin Developments Innovative solution Reduced drop heights by sliding charging and discharging processes Guaranteed change of position, optimal draining of scooping parts Low volume of coating material. High turnover of coating material Fast change of coating tank Air controlunit, so low humidity around thethe coating material

Zinc Flake Coating Stucture

ELECTROPLATING Electroplating is the application of a metal coating to a metallic or other conducting surface by an electrochemical process. Steel Part Plated Metal (Zinc) ChromateFinish ZINC PLATED PART The part is protected from corrosion by the zinc plating The zinc plating is protected from corrosion by the chromate finish There are a variety of topcoats / seals available. Silicated dips, polymers/lacquers, cross-linking polymers, lubricating seals (torque-n- tension compounds)

ELECTROPLATING

Sherardizing Sherardizing is also known as TDG(Thermal Diffusion Galvanizing). IT is a diffusion process in which articles are heated in the presence of zinc dust. The process is normally carried out in a slowly rotating closed container at temperatures ranging from C. Main Benefits Long term corrosion protection Coating thickness 15-80μm Salt spray resistance up to 800h Damage resistant Uniform coating

Sherardizing TDG coating is sacrificial. TDG is Non‐Galling, acting like cadmium. TDG coating process guarantees parts free of Hydrogen Embrittlement. TDG coating has strong adhesion to base metal due to mutual diffusion of zinc and iron. Zinc penetrates to the base metal to about 1/3 of the coating thickness..

HOT-DIP GALVANIZING (HDG) Hot-dip galvanizing is the process of immersing fabricated steel or iron into a kettle or bath of molten zinc at temperature from 430 to 560 C.

HDG In addition to barrier protection, hot-dip galvanizing protects steel cathodically, which means zinc will preferentially corrode to protect the underlying base steel.

Comparison of Coating Weight Loss in Salt Spray HDGTDG YZ

Corrosion-resistant solutions Depending on conditions, Rawlplug offers solutions meeting the demands of any application in terms of corrosion resistance.  Zinc electroplating  Hot dip galvanizing  Zinc flake coating  Stainless steel

Corrosion-resistant solutions

Salt spray test(NSS) of Zinc Flake Coating

How long each material with coating can last against corrosion Long term corrosion resistance

Stainless Steel When the selection of the stainless steel grade has not been properly made, corrosion may occur …no material is perfect! think of it as selecting the right TYPE/QUALITY for the intended use

Stainless Steel Passive Layer vs. Coatings PASSIVE FILM on STAINLESS STEEL: Oxy-hydroxides of Fe and Cr 2-3 nm thick (0,002-0,003 µm) Transparent Adherent Self repairing MILD STEEL Coat Topcoat Primer MULTI-LAYER COATINGS Typically µm thick May peel off Not self-repairing 35 Oxygen

Stainless Steel 36 Passive film Stainless Steel Self Repair Multi-layer Coating Mild Steel Corrosion Products Damage to protective layer

Stainless Steel Effect of Chromium Content on Atmospheric Corrosion Resistance (uniform corrosion) % Chromium mmpy Corrosion Rate Stainless Steels > 10.5 Cr %

Stainless Steel Major factors that influence pitting corrosion 1 (the pitting potential E pit is generally used as the criterion for pitting) 1.Temperature Increasing the temperature reduces drastically the resistance to pitting.

Stainless Steel Major factors that influence pitting corrosion 1 (the pitting potential E pit is generally used as the criterion for pitting) 2. Chloride concentration The pitting resistance decreases a the Cl - concentration increases (the log of the Cl - concentration)

Stainless Steel Pitting Resistance Equivalent Number (PREN) PREN = Cr + 3.3Mo + 16N, where Cr = Chromium content; Mo = Molybdenum content;N = Nitrogen content

Stainless Steel Critical Pitting Resistance Temperature (CPT) and Critical Crevice Corrosion (CCT) of various austenitic & duplex grades

Stainless Steel Mechanism of Stress corrosion cracking (SCC) The combined action of environmental conditions (chlorides/elevated temperature) and stress - either applied, residual or both develop the following sequence of events: 1.Pitting occurs 2.Cracks start from a pit initiation site 3.Cracks then propagate through the metal in a transgranular or intergranular mode. 4.Failure occurs

Stainless Steel Avoiding SCC – two choices Chloride induced stress corrosion cracking in standard austenitic stainless steels, viz /304(A2) or /316(A4) Select duplex grades, more price stable (less nickel) Select austenitic stainless steels with higher content of Ni and Mo (higher corrosion resistance) +Ni +Mo (6Mo) +Cr

Stainless Steel How to select the right stainless steel for adequate corrosion resistance? Eurocode 1-4 provides a procedure for selecting an appropriate grade of stainless steel for the service environment of structural members. This procedure is presented in the next slides It is applicable to: Load bearing members Outdoor use Environments without frequent immersion in sea water pH between 4 and 10 No exposure to chemical process flow stream

Stainless Steel 1.The environment is assessed by a Corrosion Resistance Factor (CRF) made of 3 components (CRF= F1+F2+F3) where a)F1 rates the risk of exposure to chlorides from salt water or deicing salts b)F2 rates the risk of exposure to sulphur dioxide c)F3 rates the cleaning regime or exposure to washing by rain 2.A matching table indicates for a given CRF the corresponding CRC class 3.The stainless steel grades are placed in corrosion resistance classes (CRC) I to V according to the CRF value The tables are shown in the next 4 slides

Stainless Steel F 1 Risk of exposure to CI (salt water or deicing salts) Note: M is distance from the sea and S is distance from roads with deicing salts 1 Internally controlled environment 0 Low risk of exposureM > 10 km or S > 0.1 km -3 Medium risk of exposure1 km < M ≤ 10 km or 0.01 km < S ≤ 0.1 km -7 High risk of exposure0.25 km < M ≤ 1 km or S ≤ 0.01 km -10 Very high risk of exposure Road tunnels where deicing salt is used or where vehicles might carry deicing salts into the tunnel -10 Very high risk of exposure North Sea coast of Germany All Baltic coastal areas M ≤ 0.25 km -15 Very high risk of exposureM ≤ 0.25 km Atlantic coast line of Portugal, Spain, France Coastline of UK, France, Belgium, Netherlands, Southern Sweden All other coastal areas of UK, Norway, Denmark and Ireland Mediterranean Coast

Stainless Steel F 2 Risk of exposure to sulphur dioxide Note: for European coastal environments the sulphur dioxide value is usually low. For inland environments the sulphur dioxide value is either low or medium. The high classification is unusual and associated with particularly heavy industrial locations or specific environments such as road tunnels. Sulphur dioxide deposition may be evaluated according to the method in ISO Low risk of exposure (<10 µg/m³ average deposition) -5 Medium risk of exposure (10 – 90 µg/m³ average deposition) -10 High risk of exposure (90 – 250 µg/m³ average deposition) F 3 Cleaning regime or exposure to washing by rain (if F 1 + F 2 = 0, then F 3 = 0) 0 Fully exposed to washing by rain -2 Specified cleaning regime -7 No washing by rain or No specified cleaning

Stainless Steel Matching Table Table: Determination of Corrosion Resistance Class CRC Corrosion Resistance Factor (CRF)Corrosion Resistance Class (CRC) CRF = 1I 0 ≥ CRF > -7II -7 ≥ CRF > -15III -15 ≥ CRF ≥ -20IV CRF < -20V (CRF= F1+F2+F3)

Stainless Steel Grades in each Corrosion Resistance Class CRC Corrosion resistance class CRC IIIIIIIVV (A2)1.4401(A4) FerriticsStd Austenitics Mo Austenitcs Lean duplexSuper Austenitics Duplex/sup er duplex Notes: Please see the appendix for EN standards designations This does not apply to swimming pools

Stainless Steel Other applications No specific regulations are applicable Grade selection must be adequate for the expected performance Three ways to do this: Ask an expert Get help from stainless steel development associations Find out successful cases with similar environments (usually available)

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