1. 铬镀工艺

刻制滚筒 → 检验（合格） → 装配 → 滚筒清洗 → 铬镀 → 抛光 → 自检（合格） → 移交最终检验（不合格，退回重新镀铬）。

2.铬镀的基本原理

铬酸在铬镀溶液中一般以重铬酸根（H₂Cr₂O₇）的形式存在，在高浓度铬镀溶液中，它可以以三铬酸根（H₂Cr₃O₁₀）和四价铬酸根（H₂Cr₄O₁₃）的形式存在。当镀液中只有铬酸而没有硫酸等催化剂时，通入直流电只会在阴极释放氢气，而没有铬层沉积，相当于水的电解。在加入适量的硫酸催化剂（CrO₃:H₂SO₄=100:1）后，阴极发生以下反应：

1) 以下反应依次在阴极发生：

Cr₂O₇^2-+ 8H^{+ 6e → Cr}+ 4H₂O₃O ①₂+ 2e → H

2H^{+ 6e → Cr}↑ ②₂+ H

Cr₂O₇^2-O → 2CrO₂+ 2H₄^2-③^{+ 6e → Cr}CrO

+ 6e → Cr↓ + 4H₄^2-+ 8H^{+ 6e → Cr}O ④₂从上述反应可以看出，铬镀的阴极反应相当复杂。现在，利用胶体膜理论和铬镀的极化曲线，我们简要描述铬镀的机制。在电化开始时，首先发生的反应是六价铬还原为三价铬（反应式 ①），如图1的极化曲线的ab段所示。随着电位向负方向偏移，电流密度急剧增加，反应①生成三价铬的速度非常快，电位偏移到点b，此时电流达到最大值。在点b之后，电位达到氢离子沉淀的点，因此反应①和②同时发生。观察极化曲线的bcd段，随着电位向负方向偏移，电流密度逐渐降低，表明电极表面的状态发生了变化。由于反应①和②消耗了大量氢离子，电极界面的pH值增加，形成一层基本铬酸胶体膜（Cr(OH)）覆盖电极表面，增加了电阻，从而降低了电流密度。阴极表面附近pH值的增加为Cr

离子转化为CrO₃离子创造了条件，导致反应③向右进行，CrO₄的浓度迅速增加。当电位偏移到点d时，此点的电位ψ对应于铬离子的还原沉淀电位，反应④开始。de段是铬镀的真实极化曲线，其中反应①、②、③和④同时发生，随着电位向负方向偏移，反应④迅速加速。在催化剂硫酸根离子的作用下，覆盖电极表面的胶体膜溶解：这种溶解首先局部发生，然后逐渐扩展，暴露出基材的小面积，导致非常高的实际电流密度和显著的极化，使铬的还原以一定速度进行。新形成的铬层表面将再次形成胶体膜，胶体膜的溶解和形成继续循环，发挥重要的调节作用。SO₂O₇^2-和在阴极生成的三价铬并不直接参与电极反应，但它们的存在和浓度对铬镀层的质量至关重要。三价铬是胶体膜的重要成分；如果其浓度低，胶体膜难以形成或形成后薄且多孔，容易被硫酸溶解，导致暴露的基材面积大，电流密度低，无法达到铬的沉淀电位，从而导致覆盖能力差；如果三价铬的浓度高，胶体膜厚且致密，难以被硫酸溶解，铬层只能在原始晶粒上生长，导致粗糙的结晶和暗淡无光的镀层。高硫酸含量容易溶解胶体膜，导致低电流密度区域没有铬层，类似于低三价铬的情况；硫酸不足则导致类似于高三价铬的情况，导致粗糙的铬层。因此，严格控制铬镀中的浓度，特别是铬酸酐与硫酸的比例是至关重要的。₄^2-ions, leading reaction ③ to proceed to the right, and the concentration of CrO₄^2-increases rapidly. When the potential shifts to point d, the potential ψ at this point corresponds to the reduction precipitation potential of chromium ions, and reaction ④ begins. Segment de is the true polarization curve of chrome plating, where reactions ①, ②, ③, and ④ occur simultaneously, and as the potential shifts negatively, reaction ④ accelerates rapidly. Under the action of the catalyst sulfate ions, the colloidal membrane covering the electrode surface dissolves: this dissolution first occurs locally and then gradually expands, exposing a small area of the substrate, resulting in a very high real current density and significant polarization, allowing the reduction of chromium to proceed at a certain speed. A colloidal membrane will again form on the surface of the newly formed chromium layer, and the dissolution and formation of the colloidal membrane continue to cycle, playing an important regulatory role. The SO₄^2-and the trivalent chromium generated at the cathode do not directly participate in the electrode reactions, but their presence and concentration are crucial to the quality of the chrome plating layer. Trivalent chromium is an important component of the colloidal membrane; if its concentration is low, the colloidal membrane is difficult to form or is thin and porous after formation, easily dissolved by sulfuric acid, resulting in a large exposed substrate area where the current density is low, failing to reach the precipitation potential of chromium, thus leading to poor coverage ability; if the concentration of trivalent chromium is high, the colloidal membrane is thick and dense, difficult to dissolve by sulfuric acid, and the chromium layer can only grow on the original crystal grains, resulting in rough crystallization and a dull, non-lustrous plating layer. High sulfuric acid content easily dissolves the colloidal membrane, leading to no chromium layer in low current density areas, similar to the situation with low trivalent chromium; insufficient sulfuric acid leads to a situation similar to that with high trivalent chromium, resulting in a rough chromium layer. Therefore, it is essential to strictly control their concentrations in chrome plating, especially the ratio of chromic anhydride to sulfuric acid.