High NH₄Cl Concentration Deposition Amorphous Ni-P Alloy Coating Study On Q235 Steel Plate Heat Exchanger Surface

Heat exchangers are used extensively in industry and our daily life. But the corrosion ofheat exchangers still can not be solved well, especially in industry. The greatanticorrosion amorphous coating can be made by means of plating and electroless. Assurface treatment technologies, plating and electroless have more advantages over othertechnologies, such as simple equipments processes, esay to control and low-cost to getcoatings and so on, and the what's more, they can be used to the complex shape parts.So they are quite suitable to solve the corrosion of heat exchangers, and they have beenused gradually.In this paper, by adding enough more ammonium and proper plating agent, usingorthogonal-design experiments, examining porosity of coating, microsurface crack,microhardness, crystal structure etc. Target of the main examine by according toporosity of coating, microsurface crack and anticorrosion ability. The processparameters and contents of plating and electroless amorphous Ni-P coating on thesurface of Q235 steel are systemically studied in low temperature, using NiSO₄ as mainsalt and reducing agent NaH₂PO₂ as P element source. Theoretic and practical basis ofmanufacturing excellent anticorrosion amorphous Ni-P coating on Q235 steel surface inthe low temperature is made in the paper. The regulars of electroless amorphous Ni-Pcoating on the surface of Q235 steel in high ammonium condition are also studied. Thestudy results as follow:1. The factor effect regulars of Electroplating Ni-P amorphous alloy coating are asfollows:â‘ NH₄Cl amount: XRD test showed that the structure of coating is amorphouswhen the amount of NH₄Cl is over 0.6mol/L; the more of NH₄Cl amount, themore serious the coating porosity and crack is;â‘¡Effect regular of PH: coating can't be made when PH value is low 2, butbetween 3 to 4; PH has great effect on coating's porosity and crack: the lowerPH is, the lower the porosity is, but the crack of the coating is more serious. â‘¢Temperature T: temperature has little effect on porosity and crack of coating,when it betweens 25 degree to 45 degree. However when the temperature isover 45 degree, the temperature is higher, the porosity is lower, and the crack isless serious, when the temperature is high up to 75 degree, the crack can bedisappeared.â‘£Current densityâ… : the higher current density is, the more porosity will be;porosity is lowest and crack is lest when current density is 1.5 A/dm²;2. Under the condition of no additives, corrosion resistance of the amorphous coatingmanufactured by high concentration of NH₄Cl is bad because of it serious crack andhigh porosity. Ni-P amorphous coating corrosion resistance is improved greatlyafter 2 times orthogonal experiments by adding additives of saccharin, sodiumdodecyl sulfate and 1, 4 butynediol properly. Optimal formulation and processes asfollow:NiSO₄·6H₂O=40g/L, NaH₂PO₂·H₂O=30g/L, H₃BO₃=15g/L, NH₄Cl=32g/L, saccharin=0.12g/L, sodium dodecyl sulfate=0.03g/L, 1, 4 butynediol=0.1 g/L, temperatureT=45℃, current densityâ… =1.5 A/dm²;3. EDS analysis of the coating made by optimal formulation and processes certificated:the courses of electroplating and chemical plating Ni-P amorphous are dynamic. Inelectroplating course, the more close to the matrix, the lesser of P content in thecoating; from surface to the centre, and from the centre to the part near to the matrix,the weight percentage of P ranges 6.61%→7.18%→8.73%, the deposition velocityis 25.02um/h; In the course of chemical deposition, P element is fluctuating changedin the coating, from surface to the centre, then from the centre to the part near to thematrix, P element weight percentage ranges 10.79%→10.97%→10.70%, thedeposition velocity is 9.00um/h;4. The regulars of electroless Ni-P amorphous coating almost keep the same as theregulars of electroless Ni-P amorphous coating without the high concentration ofNH₄Cl. The influence laws of temperature and PH in chemical plating Ni-Pamorphous coating under the high concentration of NH₄Cl are the same as thecommon chemical plating Ni-P amorphous coating laws; 5. plating temperature, pH value and performance additives for Electroless Ni-Pamorphous alloy coatings effects are as follows:â‘ temperature T: The higher the temperature, the faster the plating velocity, thehigher the coating hardness, and the lower the porosity, so the better corrosionresistance, However, the poorer the stability of the bath;â‘¡PH value: The higher the pH value, the faster the plating velocity, and thehigher the coating hardness, but the worse the bath stability; the bath stability;take the plating velocity and the corrosion resistance into account, experimentcame to an good conclusion: NiSO₄·6H₂O=40g/L,H₃BO₃=15g/L,NaH₂PO₂·H₂O=30 g/L,NH₄Cl=0.6 mol/L,PH=4,T=85℃;â‘¢Additive: the more, the Additives, the slower the deposition velocity, the higherthe stability of the bath; It can be appropriate to reduce the hardness of thecoatings, reduce porosity. It makes anode polarization curve has broaderpassivasion, lower passivasion voltage and passivasion current in secondpassivasion, improve coating corrosion resistance; among the three additives,the amount of saccharin has the greatest effect on bath, the second one is 1, 4butynediol's, and the last one is sodium dodecyl sulfates; taking the stability,deposition velocity and anticorrosion property into account, the optimumprocesses and formula is as follow: NiSO₄·6H₂O=40g/L,H₃BO₃=15g/L,NaH₂PO_·H₂O=30g/L,NH₄Cl=0.6mol/L,PH=4, T=85℃,saccharin=1.0 g/L,sodium dodecyl sulfate=0.15g/L,1, 4 butynediol=0.3 g/L;6. Anode polarization curves tests show: Q235 steel corrosion resistance has greatlyimproved after low temperature electroplating and electroless amorphous Ni-Palloy coating; electroless amorphous Ni-P do better. Ni-P alloy coating makesQ235 steel enters a stable passivation district at the beginning stage in wt%=42%LiCl solution; Q235 steel appears second passivation and third passivation in theanode polarization curves because of the P fluctuation in the coating.