Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís and Doníts of Dealing with Stress Corrosion Cracking.

Electroless nickel plating can prevent stress Corrosion cracking (SCC) in acid or alkali environments; How? See below.

Stress corrosion cracking in metals, polymeric materials and ceramics occurs when internal or external stress is present along with moisture, condensates, or in bulk solutions and corrosive materials such as chlorides or strong alkalis are present. Cracks can occur slowly or cataclysmic rapid fractures. Cracks occur when the stress exceeds the fracture strength of the material. SCC is often delayed or act at a slow rate depending somewhat on environmental conditions. Cracks occur when the stress exceeds the fracture strength of the material. Some convincing data points to a roll for hydrogen in SCC. Corrosion can take place in either an acid or alkaline exposure. In acid environments it is postulated that hydrogen absorption (hydrogen embrittlement) is the cause of SCC. In an alkaline environment it is postulated that there is dissolution of metal combined with intrinsic or external stress.

Aqueous chlorides are the most corrosive and are found everywhere, sea water, and airborne sprays near the sea. Chlorides are present in numerous products, food, natural waters, steam condensate, coffee, baked beans, tomato soup, refrigerants, lubricants, tapes, ink, adhesives, soils, etc.

The lower the pH the faster is the corrosion process and the release of hydrogen to cause hydrogen embrittlement. Oxygen promotes hydrogen stress corrosion cracking

The second most active in SCC on austenitic stainless steel is sodium hydroxide. Cracking scan be transgranular and intergranular and is a function of concentration and temperature, but seldom below 100C

Prevention Protective coatings (free from chlorides), Zinc plating, zinc rich coatings act as sacrificial materials. Barrier coatings such as electroless nickel(EN), nickel electroplating, hard chrome, or other specialized coatings that do not contain chlorides or bromides can protect. One of the most effective barrier coatings is compressive deposits of electroless nickel in acid and/or chloride exposure. High phosphorus electroless nickel has compressive stress.  High P EN provides the best corrosion barrio of all the non-sacrificial coatings.

Phase studies show that EN coatings are met stable as plated with three separate non-equilibrium phases (alpha, beta and gamma). The presence of and amounts of each phase is dependent on the total amount of phosphorus in the deposit. Below 4%P the beta phase is dominant. This deposit shows the best corrosion protection in alkaline conditions. Above 4.5% to 11% the alpha phase increases and the beta diminishes.
There higher P content EN deposits are protective in acid environments. Above 11% Phosphorus content the structure is a homogeneous gamma phase. These deposits are resistant to acid, organic acids, chloride and bromide environments

Doís

Use electroless nickel plating to prevent stress corrosion cracking of high strength steels and stainless steels. High Phosphorus deposits for acid, chloride or bromide exposure. Low Phosphorus deposits for alkaline exposure.

For the best corrosion protection the surface to be plated must free of roughness, pits, metal splinters or sharp edges and cleaned and deoxidized.

Make frequent additions to maintain the best plating conditions.

Keep temperature and pH steady.

Keep dust and particulate matter out of the solution.

Use adequate filtration, and cover the tank when not in use.

Select the electroless nickel phosphorus content for the intended environment

Choose the correct pretreatment for the alloy to be plated; Woods or Sulfamate electroplating strike for stainless steels. Sulfamic acid, citric acids can be used for pretreatment.

Shot peen high strength steels to lower the tensile stress at the surface. Use clean shot

Doníts

Use bare hands on surfaces to be plated.

Allow EN solution to get out of balance chemically.

Make large additions at one time.

Use hydrochloric acid for preparation of high strength alloys.

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